This document provides an overview of gemstones, including their physical and optical properties, shapes and cuts, and synthetics and simulants. It discusses the key properties used to identify gemstones such as crystal habit, specific gravity, hardness, refractive index, and luminescence. Different cutting styles including cabochon and faceted cuts are described. The document also covers synthetic gemstones which mimic natural stones, and simulants which have similar appearances but different properties. It aims to equip gemologists with the knowledge to distinguish natural gems from synthetics or enhanced stones.

1. Contents
1) An Introduction to Gemstones
2) Shapes and cuts of gems
3) Physical and Optical properties of Gemstones
3.1.1. Crystal habit/Form/System
3.1.2. Specific Gravity
3.1.3. Hardness
3.1.4. Cleavage
3.1.5. Fracture
3.1.6. Thermal and Electrical conductivity
3.2.1. Color
3.2.2. Luster
3.2.3. Physical optical effects
3.2.4. Transparency
3.2.5. Refractive Index
3.2.6. Luminescence
3.2.7. Pleochroism
3.2.8. Optic sign and other optical properties
3.2.9. Spectrum analysis
4) Inclusions and internal features of natural Gemstones
5) Synthetic Gemstones
6) Enhancement of a gemstone
7) Popular Gemstones, their synthetics and simulants.
 Diamond
 Corundum
 Beryl
 Garnet
 Tourmaline
 Cat’s Eye and other chatoyant gems.
 Topaz
 Quartz, Tiger’s eye, Chalcedony and Opal
 Jade
 Lapis Lazuli
 Organic Gemstones
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2. CHAPTER - I
An Introduction to Gemstones
A gemstone is a mineral or an aggregate of a mineral which, due to one or more optical
properties, can be rendered sufficiently beautiful though cutting and polishing, so as to be used
in jewelry or other decorative purposes. The cut and polished final product produced from the
uncut gemstone is called gem.
Exceptions to this general definition are the organic gem materials like pearl, amber, coral and
jet, which, although products of nature, are not strictly minerals. Similarly, Lapis Lazuli, one of
the oldest known gemstones to be used by man, is not strictly a mineral but a rock dominantly
constituted of the mineral lazurite with certain proportions of sodalite, pyrite and even calcite.
The principal qualification of a gemstone lies in its beauty which can be rendered by one or
more of the optical effects including color, transparency, dispersion, luster, chatoyancy,
iridescence etc. Skillful cutting brings out the beauty of a gemstone to its full extent and
therefore the valuation of a gem is also dependent on its cutting. Durability is another important
property. Because most gemstones are used for personal adornment, they should be able to resist
abrasion. However, only 10 to 12 gemstones have hardness equal to or more than 7 in Mohs
scale. Rarity renders value and carat weight (caratage) adds to it. Finally, the desirability of a
gem is also subject to trends of fashion. For example, the dark red pyrope garnet was much in
vogue during the nineteenth century but is rarely in demand today.
Gemstones have ever been in demand since the beginning of civilization. In ancient Egypt lapis
lazuli had been in great demand for amulets and ornaments such as scarabs and the Ankhs. Lapis
jewelry has been found through excavations at the predynastic Egyptian site of Naqada (3300-
3100 B.C.). Powdered lapis was used as eye shadows by members of the Egyptian royal
dynasties. The ancient Sumerian tombs of Ur, located near the river Euphrates in lower Iraq
contained more than 6000 lapis lazuli statuettes of birds, deer and rodents as well as beads and
seals. The Sumerian-Akkadian epic of Gilgamesh makes repeated references to this gem along
with precious metal like gold, where, for instance, the ‘Bull of Heaven’s’ horns were made of
Lapis. Beads of lapis have been reported from Neolithic burials in Mehrgarh and other localities.
It was also popular amongst the men of ancient Indus valley who had set up the Harappan
colony of Shortugai near the Lapis mines of Afghanistan. Similarly, carnelian, chalcedony,
jasper and agate are semiprecious gemstone that has been in use since Neolithic times. Ancient
Egyptians also knew of quartz, turquoise, olivine, fluorite and malachite. Diamond, the king of
gems, was known to the ancient Indians even 2500 years ago. Many of the world famous
diamonds like Koh-i-noor, Great Mughal, Orloff, Regent and Hope have their origin in India.
The “Syamantak” of Mahabharata has been believed by many to be a diamond (some even
speculating, evidently without the slightest historical evidence, it to be one and the same as the
famous Koh-i-noor). By the time of the ancient Greek and Roman civilizations, many of the
gem mineral were known and the art of carving them was well understood. The people not only
used gems for personal adornment but their importance increased in presumed supernatural
powers endowed upon the wearers. They were believed not only to protect the wearer from
disease, fire, poison, intoxication, lighting and other disasters but also bestowed strength, power
2

3. and wisdom. Thus, gemstones have been ever in demand for beauty, as expression of power and
wealth and for other supernatural beliefs.
Due to this ever increasing demand for different varieties of gems, the market is flooding with
simulants, synthetics and composite stones alongside natural gems. Color enhancement of low
quality gemstone is also a widely applied process from ancient days and the techniques have
been further and further refined through advancement of technology so much so that extremely
careful scrutiny is now required to separate a natural, good quality gem from a synthetically
prepared or a poor quality color enhanced equivalent.
A Synthetic gemstone is basically an artificially prepared material having composition, crystal
system, physical and optical properties comparable to its natural equivalent. Except for
gemstones with very complex chemical compositions (viz. tanzanite, topaz, tourmaline etc),
most important gemstones can now be synthesized by various techniques. Among the colored
stones, the most important synthetics are that of ruby, sapphire, emerald, amethyst and spinel.
They closely resemble the naturals but have certain subtle differences to distinguish which the
gemologists are called upon. Synthetic diamonds, grown from a metallic flux at high
temperatures and pressures, evoked much concern in jewellery trade ever since the first
production of gem quality crystals in 1970s. However, since the cost is still too great to produce
it industrially, the chief threat to diamond is not these synthetic products but a variety of
diamond simulants.
A stimulant can be any material, natural or synthetic, which has the external appearance of the
gem it imitates but quite different physical and optical properties. For example, gem quality
kyanite (natural mineral) has frequently been used as a stimulant of blue sapphire. Similarly,
synthetically produced blue spinel also forms a common stimulant of blue sapphire. Cubic
zirconium oxide (CZ) is an artificially produced gem (without any natural equivalent) that has
been sold both as a stimulant of diamond and on its own accord as American Diamond. Many
historically famous gemstones have turned out to be simulants, one of them being the famous
emerald Buddha of Thailand which is actually jade.
Composite gems are tricky things. A doublet composite is constituted of two parts cemented
together in such a way that it resembles a single larger gemstone. It generally has a crown made
up of natural gem and a pavilion of glass or synthetic or stimulant. Careful observation under
microscope can reveal the line of joint or perhaps a bubble along the cementing material. A
diamond topped doublet made from thin crown section of diamond cemented to a pavilion of
quartz or some diamond stimulant has even deceived expert jewelers. A triplet has three parts.
For example, a ‘soude’ emerald is a triplet consisting of synthetic beryl/spinel/ colorless quartz
at the top and base and a thin layer of coloring material between the crown and the pavilion
(girdle area) making up the third component of the triplet.
Imitation gems are cheap low quality materials, generally glass, that are made to imitate the
natural ones in color and luster but are otherwise completely different.
3

4. In contrast, an enhanced gem is a natural low quality gemstone that has been somehow treated
to enhance its color or to impart color. The enhancement of poor quality gemstone is an ancient
art developed gradually with advancement of technology. It includes processes as simple as
dyeing or using mirror backs to as complex as bombardment of the gemstone by charged
particles like electrons or neutrons.
In view of all these possibilities it becomes necessary for a gemologist to understand the true
nature of a gemstone and its simulants and synthetics. Fortunately, a number of instruments are
now available, which when used with proper knowledge backup, can help a person to decipher
the true character of the gem. The following sections therefore provide a brief review of the
instruments that are commonly used in a gemological laboratory. Also the characters of some of
the commonly used gemstone have been discussed along with their simulants and synthetics.
However, as it is that gemology is an ever researched subject and new synthetics, better
composites and finer enhancement techniques are constantly coming up, the details of which
remain jealously guarded, it is advisable to constantly update oneself in this changing, varied
and extremely interesting field of geology.
CHAPTER - II
Shapes & Cuts of Gems
Gem cutting is the art of fashioning a rough stone into a sparkling gem. Basically two broad
styles of cutting are recognized: (1) cabochon cut and (2) faceted cut.
Cabochon cut is the most primitive style of cutting that can be traced back to Protohistoric
times. This kind of cutting is presently applied for opaque and translucent stones like turquoise,
lapis lazuli etc. Also the gemstones having special optical properties like chatoyancy (eg. Cat’s
eye, tiger’s eye etc), asterism (diopside, star ruby etc), opalescence (eg. Precious opal) or
labradorence (labradorite) are fashioned as cabochons. Primarily three types of cabochons are
recognized (Fig 2.1):
(a) Simple cabochons: Stones cut into a plainoconvex shape with domed top and flat base
(Fig 2.1a).
(b) Double cabochons: Stones cut into biconvex shape, usually the upper dome is cut with
greater convexity (Fig 2.1b).
(c) Hollow cabochons: Stones cut into a concavo-convex shape (Fig 2.1c). Generally used
for deeply colored transparent to translucent stones for increasing light transmission
through decrease in thickness.
Circumference of a cabochon stone may be circular, oval, polygonal or pear shaped, heart
shaped etc.
4

5. Faceted cut is the most common style employed for fashioning transparent gemstones. Two
fundamental faceting styles can be recognized: (a) step cut and (b) brilliant cut (Fig 2.2a). There
are further variations within these two styles of cutting which mainly depend on the shape of
rough stones. These include shapes like marquoise-, baugette-, lozenge-, oval- and pendeloque
(Fig 2.2a). There are also mixed cuts. Additionally stones are cut into a few
other shapes like briolette, star-shape and heart-shape. Step cut is widely employed on emerald
and stones of prismatic habit.
Essential parts of a feceted stones are: (i) crown and (ii) pavilion (Fig 2.2b). The crown, also
called bezel or top and consists of a large centrally placed horizontal facet called table and many
polygonal facets. The pavilion, also known as the base or back, usually comprises of elongated
triangular or kite shaped facets. Culet is an additional small facet often cut at the tip of the
5

6. pavilion for preventing the stone from further damage and allowing some amount of light to
pass through the stone directly. This facet, so popular about a century back, is more or less
obsolete at present. The junction of the crown and the pavilion is called girdle. Fig 2.2b
diagrammatically presents the different facets of a gem.
CHAPTER - III
Physical and Optical properties of Gemstones
Gemstones are extremely costly and therefore for gem testing no chemical test can be applied. A
series of physical and optical examinations are hence performed in gem testing laboratories in
order to identify a particular stone.
3.1. Physical properties of Gemstones
The following is a brief description of the physical properties of gemstones that are commonly
studied for identification purpose.
3.1.1. Crystal Systems and habits
Crystal habits, dependent upon the crystal systems and forms, when combined with color, serve
as a useful means to form an idea about the nature of an uncut raw gemstone. Table 3.1 gives an
idea about the crystal habits and systems of some commonly used uncut gemstones while Plate-
6
Fig 2.2b. The different facets in standard
brilliant cut (from Geological Society of India,
Memoir 45)
Proportion of cutting
A particular proportion of cutting is essential to bring
out the brilliance and fire of a stone. It includes (a) ratio
of lengths of crown and pavilion, (b) angles of crown
facets and pavilion facets and (c) proportion of table
facet to the rest of the facets. Depends upon the critical
angle of reflection for the gem material concerned, the
facets are angled and proportioned in such a way so as
to obtain total internal reflection from the stone. Ideal
proportions for modern brilliant cut diamonds have
been given in Fig 2.2b.

7. 1 provides photos of a range of crystals demonstrating the widely varying forms and colors of
common gemstones.
Table 3.1. Crystal systems and habits of some commonly used gemstones
Gemstone Crystal system Habit
Diamond Isometric Octahedron, dodecahedron, icoaitetrahedron (cubes rare),
contact twin common (macles)
Corundum
(Ruby,
Sapphire)
Trigonal Ruby: tabular hexagonal prism; Sapphire: tapering barrel
shaped bipyramid
Beryl
(Emerald,
Aquamarine)
Hexagonal Six sided prism, often striated vertically
Apatite Hexagonal Six sided prism, often terminated with pyramid (also tabular
prism and massive)
Kyanite Triclinic Bladed aggregate
Garnet Isometric Dodecahedron, icosatetrahedron (and combination of both)
Topaz Orthorhombic Flattened four sided prism with pyramidal or dome
termination (prism face often vertically striated)
Spinel Isometric Octahedron and spinel twin (contact twin)
Tourmaline Trigonal Triangular prism (generally with rounded faces, heavily
striated along length)
Turquoise Triclinic Massive
Zircon Tetragonal Four sided prism with bipyramidal terminations
Calcite Trigonal Rhombic prisms, scalenohedra and six sided prisms
Chalcedony Trigonal
(microcrystalline)
Massive, botryoidal, mammilary, nodules
Chrysoberyl Orthorhombic Prismatic crystals, triple contact twins forming ‘hexagon’.
Quartz Trigonal Six-sided horizontally striated prism with rhombohedral
terminations
Rhodochrosite Trigonal Massive
Rhodonite Triclinic Tabular and massive
Rutile Tetragonal Four sided prism with pyramidal terminations (also acicular
and massive granular)
Scapolite Tetragonal Four sided prism (also massive)
Felspar
(Orthoclase)
Felspar
(Microcline
and
plagioclase)
Monoclinic
Triclinic
Crystal of both types resembles each other in habit; both are
prismatic and blocky with wedge shaped faces.
Fluorspar Cubic Cube, interpenetrant cubes and octahedral crystals
(naturally occurring octahedral are rare, but the cubes
7

8. cleave readily into this form
Peridot Orthorhombic Prismatic
Pyrite Cubic Cube, dodecahedron (also massive and granular form)
3.1.2. Specific Gravity
Specific gravity of a substance is the ratio of its weight in air to the weight of an identical
volume of pure water (at standard atmospheric pressure and 4°
C- the temperature at which water
is most dense). As specific gravity is a ratio, it has no unit of measurement. Determination of
specific gravity is one of the most effective and commonly used modes of gemstone
identification since specific gravity of each mineral is either a constant or can vary within a very
short range depending upon the substitutions. It is equally applicable to both cut and uncut
gemstones.
The most common practice of measurement of SG is by hydrostatic method. Hydrostatic method
is based on the principle that an object immersed in a liquid experience an upward force
(buoyancy or loss of weight) equal to the weight of the displaced fluid. The method includes
weighing of gemstones in air and then weighing it again when completely immersed in pure
water.
The SG of a specimen is determined as:
SG of Gem = weight of gem in air / weight of displaced pure water
= (weight of gem in air) / (weight of gem in air – weight of gem in water)
Specific gravity of a gemstone is determined with the help of a single pan electronic balance
with accessories fitted for specific gravity determination (Plate-2).
Precaution in measurement of specific gravity:
 Errors may be introduced by air bubbles adhering to the gemstones and by surface
tension of the water (cause friction like drag on the stone holder). Bubbles can be avoided
by thoroughly wetting the specimen before immersion. Residual bubbles can be removed
by camelhair paint rush.
 Mixing a drop of detergent with water can reduce surface tension effect.
* A list of specific gravities of important gemstones in the order of increasing specific gravity
has been provided in Appendix-IB.
3.1.3 Hardness
Hardness of a material is defined as the ability of that material to resist abrasion. Comparative
values of hardness in Mohs scale for minerals are available in all standard mineralogical
textbooks and form an important property for identification of rough, uncut stones. However,
hardness test for cut and polished specimen should, if at all, be done with much more precaution
since any abrasion with harder material will leave a permanent scratch mark on the polished
8

9. surface. However, synthetic corundum pieces, kept in the laboratories, are sometimes used as
scratch plates for testing suspected diamonds.
Hardness renders durability although it is not the sole controller of the later. There is certain
other lesser important qualities like brittleness and toughness, which contributes to the durability
of a stone. Zircon, for example, has a hardness of about 7 – 7.5, but is brittle, as a result often
suffers chipping. Similarly, diamond, even after being the hardest known mineral is brittle
because of its cleavage planes. On the other hand, two jade minerals nephrite and jadeite have a
considerable degree of toughness, despite their hardness values of 6 and 7 respectively. They do
not represent single crystals, but a mass of microscopic interlocking fibers or crystals. As a
result, they can withstand much more wear and tear than the harder mineral zircon.
3.1.4. Cleavage and Parting
Cleavage is a set (or multiple sets) of equally spaced planes in a crystal along which the atomic
bonds are much stronger compared to the bonds across the planes. As a result, the crystals
(gemstones in present case) are easily cleavable along the cleavage planes. This property is
usable both for identification of raw gemstones and for cutting and shaping of hard gemstones.
Cleavage renders a gem brittle. For example, with a sharp strong blow, even diamond, the
hardest of all known minerals, is cleavable along its octahedral cleavage planes. The minerals
with very well developed cleavage (e.g. diamond, topaz etc) need special care during lapidary.
In contrast to cleavage, parting planes are more widely spaced and less regular. Gemstones like
corundum and labradorite have parting planes along the planes of repeated lamellar twinning.
3.1.5. Fracture
The way a gemstone breaks in a random direction (other than cleavage and parting) is called
fracture. Many gemstones have conchoidal fracture pattern (e.g. quartz, garnet etc) but some
have hackly or splintery fracture pattern (e.g. nephrite, jadeite, ivory). In polished and faceted
kyanite, the cross fractures are often identifiable under microscope or through a lens and serves
as an important identifying property in addition to specific gravity and refractive index.
3.1.6. Thermal and Electrical Conductivity
Diamonds are very good conductors of heat while most diamond simulants (except for
moissanite) are poor conductors. At room temperature, the thermal conductivity of Type I
diamond is about 1000 watts/m/°C and Type IIa is 2600 watts/m/°C. In contrast, the most
common diamond stimulant CZ (Cubic Zirconia) has a conductivity of only 10 watts/m/°C and
white sapphire (another stimulant) has 40 watts/m/°C. This difference in conductivity can be
used for identification of diamond. Conventional diamond testers (thermolysers, Plate-2)
differentiate between diamonds and most simulants on the basis of this difference in thermal
conductivity.
9

10. A conventional diamond tester (battery or electricity operated) has a metal tip that gets hot and
measures the amount of heat loss from the tip when it is applied to a substance. When this tip is
applied to a diamond, the diamond takes the heat from the tip and the tester will indicate
diamond. A diamond tester is able to distinguish the lesser conductors (simulants) from
diamonds. However moissanite being a high heat and electricity conductor diamond stimulant,
needs others tests.
Some gemtones, notably amber, diamond, tourmaline and topaz generate static electricity when
vigorously rubbed with dry silk and become capable of attracting small bits of paper.
Tourmaline and untwined quartz have pyroelectric property i.e. develops opposite charge at the
ends of c-axis when heated. However, this property is not exhibited by the common schrol
variety of tourmaline. Piezoelectricity (charge developed subjected to mechanical compression)
is found in quartz and has got several industrial uses. However, such electrical properties,
although of industrial uses (e.g. quartz watches) are not commonly used by gemologists.
Electrical conductivity is of interest to gemologist in the exceptional case of certain diamonds.
This mineral is normally a good insulator but Type IIb diamonds have lattice imperfections with
loose electrons. A potential difference applied to such a diamond will cause a migration of
electrons to the positive pole. This flow of electrons is small at first, only a few milliamperes,
but the flow tends to heat up the crystal which liberates more and more electrons from the atoms
and the current increase rapidly to several amperes. All natural blue diamonds are Type IIB and
therefore electroconductive whereas blue diamonds created through enhancement by irradiation
in high energy electrons in an electron accelerator are nonconductive. Therefore, this property is
frequently used to separate natural blue diamond from enhanced ones.
3.2. Optical Properties of Gemstones
Any property of a gemstone that is dependent on light is an optical property. In absence of
chemical tests, the optical properties of the gemstones, particularly polished gems, need to be
carefully studied for identification purpose. The following is a list of the different optical
properties commonly studied in gemological laboratories.
3.2.1. Color
Color is the most attractive property of a gemstone although generally ineffective for
identification purpose. For example, quartz can be colorless, transparent (rock crystal), purple
(amethyst), pink (rose quartz), brown (smoky quartz) and even green depending upon the
impurities present. Similarly, depending upon small amounts of transition ion impurities,
corundum can be red (ruby), pink (pink sapphire), blue (blue sapphire), yellow (yellow
sapphire), green (green sapphire) and even colorless (white sapphire). Understanding the cause
of coloration of crystals is therefore an important and interesting branch of mineral physics. For
present purpose, only a brief idea about the different causes of coloration of gemstones has been
given. The gemologist should do better not to identify a gemstone only on the basis of its color.
Plates 1 & 3 shows the wide variation of colors encountered in the world of gemstones.
Causes of coloration of gemstone
10

11. When white light (wavelengths from ~ 400 to 700 nm) is incident on a gemstone, it may be
transmitted, scattered, reflected, refracted or absorbed. If the light suffers no absorption, the
mineral is colorless. Minerals appear colored when certain wavelengths of light are absorbed.
The perceived color results from the combination of those remaining wavelengths that reach the
eye.
The electronic processes responsible for light absorption and color are as follows:
(i) Crystal Field Transition
Crystal field transitions are electronic transitions between partially filled 3d orbitals of transition
elements. These are the elements of the first transition series with atomic numbers from 22 to 29
and having electronic configuration of the general form 1s2
2s2
2p6
3s2
3p6
3d10-n
4s1-2
. The
electrons in the partially filled d-orbitals can be excited by quanta of energy from the visible
spectrum. Such electronic transitions are the basis for production of color. The transition
elements are therefore called chromophores.
When such chromophoric transition element/elements are the major constituents of the
composition of a crystal/gemstone, then the minerals are described as idiochromatic.
e.g. The red color of almandine garnet (Fe3Al2Si3O12) is attributed to Fe+2
while the blue color of
turquoise [CuAl6(PO4)4(OH)8.5H2O] is due to the dominance of Cu+2
.
In contrast, gemstones are allochromatic when their color is attributed to small amounts of
chromophores present as impurities within the crystal structure. Even less than 0.01% of such
elements would be enough to produce an appreciable perception of color. Table 3.2 lists the
different gem varieties of allochromatic corundum and the relevant chromophores while Table
3.3 is a list of the transition elements in order of increasing atomic number and the minerals
(idiochromatic or allochromatic) to whom they have imparted the color.
Table 3.2. Gem varieties of corundum and their chromophores
Gem (Corundum) Color Chromophore
1. White Sapphire
2. Ruby
3. Blue Sapphire
4. Yellow Sapphire
5. Padaparadscha
White
Red to pink – red
Blue
Yellow
Orange – pink
Pure
Cr+3
Ti+4
– Fe+2
Fe+2
– Fe+3
Fe+3
+ Fe+2
+ Cr+3
Table 3.3. Transition elements and minerals they have colored
Titanium (Ti) Blue sapphire (with iron), blue zoisite
Vanadium (V) Grossular garnet (Tsavorite), green vanadium beryl,
synthetic corundum (alexandrite simulant), synthetic
emerald (few), blue/violet sapphire
Chromium (Cr) Ruby, emerald, red spinel, pyrope garnet, chrome grossular
garnet, demantoid garnet, uvarovite garnet (idiochromatic))
chrome diopside, green jadeite, pink topaz, alexandrite,
hiddenite (spodumene)
11

12. Manganese (Mn) Rhodochrosite, rhodonite, spessartine garnet (all
idiochromatic), rose quartz, morganite (beryl, andalusite)
Iron (Fe) Sapphire, sinhalite, peridot, almandine garnet (all
idiochromatic), aquamarine, blue-green tourmaline,
enstatite, amethyst
Cobalt (Co) Synthetic blue/green spinel, synthetic blue quartz, cobalt
glass. Except for rare blue spinel is not found in any natural
transparent gemstone.
Nickel (Ni) Chrysoprase (green chalcedony), synthetic green and
yellow sapphire.
Copper (Cu) Malachite and Turquoise (both idiochromatic), diopside,
synthetic green sapphire
The amount of energy absorbed depends not only on the chromophores but also on their
positions in the lattice and the resultant nature and degree of crystal field splitting. For example,
the red color of ruby and the green color of emerald are both caused by small amounts of Cr+3
replacement at the Al+3
site. But since in emerald the Al+3
(or Cr+3
) are shared with BeO4 and
SiO4 tetrahedra while corundum consists of hexagonal close-packed layers of oxygen, Al+3
(or
Cr+3
) occupying interstices between the layers, the nature of crystal field splitting and hence
energy absorption of Cr+3
is quite different for emerald and ruby. Hence the stark difference in
color.
Also, Color of some minerals differs depending on the source of light. This phenomenon is
known as alexandrite effect. Best example is alexandrite a special variety of mineral chrysoberyl
(BeO, Al2O3), from which the effect derived its name. The chromophore in case of alexandrite is
Cr3
that has an absorption scheme intermediate between those of ruby and emerald. In
incandescent light (more yellowish), its colour is red and in evenly balanced light or in day light
it appears green (ruby in night & emerald in day). Alexandrite showing conspicuous color
change is much more valuable than diamond of same weight. This effect is also revealed by V+
bearing corundum (bluish in daylight, purple in incandescent light). The colors of some
commonly used natural and synthetic gemstones have been tabulated in Appendix-II.
(ii) Molecular Orbital Transition / Charge Transfer Transition
Molecular orbital transition occurs in minerals when valence electrons transfer back and forth
between adjacent ions. The electrons are thus contributed to shared molecular orbitals and are as
such delocalized. An well known example is the transition of electron between Fe+2
→ Ti+4
which is responsible for the blue color of sapphire. The Fe-Ti transitions in sapphire absorb
wavelengths in the green, yellow, orange and red range of the spectrum making the crystal
appear blue in color. Also there can be simultaneous electron transfer between Fe+2
– Fe+3
and
Fe+3
– O-2
which absorb energies in infrared and violet-ultraviolet portions of the spectrum
respectively.
(iii) Color Centers
12

13. Coloration can be caused by structural defects. This can be an excess electron that is unattached
to any single atom and trapped at some structural defect like a missing ion. This can also be an
electron missing from its side forming a hole. Such a single electron is bound in place by the
crystal field (electrical field) of all the surrounding ions. Under such conditions it can occupy a
ground state and several excited states thereby causing color and fluorescence. Gemstones
colored by this include fluorite, amethyst, blue topaz and colored zircons. The smoky color of
quartz is attributed to a ‘hole color center’.
(iv) Presence of impurities
Mechanical admixture of impurities can cause color. For example, finely dispersed chlorite or
fuchsite can make quartz appear green. Jasper appears reddish brown due to mechanical
admixture of fine hematite.
(v) Presence of organic substances
The colouration of organic gems like amber, coral etc are due to presence of dye like organic
substance in them.
(vi) Band Theory
In crystal like diamond, galena, pyrite there is covalent bonding, which involves sharing of
electrons between atoms rather than a transfer. The colour of these materials (apart from those
impurities) involves the ‘band theory’/’band gap colours’, and depends upon the energy gap
between the ‘conductive band’ and ‘valency band’ rather than between energy levels of single
atom. In case of diamond there is a wide gap between these levels in terms of energy and the
crystal is essentially colorless and does not conduct electricity. Introduction of other atoms into
the diamond lattice can cause narrowing the width of band gaps. This allows absorption of
certain parts of visible light and as a result diamond shows color.
3.2.2. Luster
The term luster refers to the general appearance of the mineral surface in reflected light. There
are two types of luster: metallic and nonmetallic. In minerals with metallic bonding, the energy
gaps between the ground state and excited states of electrons are generally much smaller than
those for ionic and covalently bonded substances. The energy of visible light is generally much
smaller than the energy gaps in ionic and covalent structures. In metallic compounds, however,
there are large numbers of excited states with energies that are available in the entire range of
the visible spectrum. This means that any quantum of energy striking the surface of a metal or
partially metallic bonded crystal is absorbed and immediately re-emitted as visible light. This
results in the typical metallic luster where light is reflected almost completely.
The ionic and covalent bonded crystals normally have non-metallic luster. Depending upon the
nature of luster they are classified as vitreous, sub-vitreous, resinous, greasy, silky, adamantine,
velvety and dull.
3.2.3. Physical Optical Effects
13

14. Under this category are classified a number of optical effects involving reflection, dispersion,
scattering, interference etc.
Iridescence: Interference of light in the interior of a mineral may produce a series of colors as
the angle of incidence changes (Opal in Plate-3). This is caused by the presence extremely thin
layers or regular structures beneath the surface of a gemstone (e.g. presence of millions of
regularly arranged submicroscopic spheres (equal sizes) of cristobalite or silica gel in precious
opal). These layers or structures act as diffraction gratings for white light, enhancing some color
and canceling others depending upon the interplanar spacing, wavelength of light, refractive
index of mineral and angle of incidence.
Opalescence: Sometimes irregular internal structures lead to scattering of light within a mineral
producing a milky appearance known as opalescence.
Labradorescence: Iridescence caused through light scattered by extremely fine (less than 1/10th
micron or thinner in width) exsolution lamellae in the range of An47 to An58 in labradorite
(labardorite in Plate-3).
Adularescence: Also known as ‘shiller’, a bluish sheen, seen in the moonstone variety of
feldspar.
Chatoyancy: In reflected light some minerals have a silky appearance which results from
closely packed parallel fibers (acicular growth) or from a parallel alignment of inclusions or
cavities. When a cabochon gemstone is cut from such a mineral / mineral aggregate, it shows a
band of light at right angles to the fibers or direction of inclusions. This property is known as
chatoyancy and is particularly impressive in cat’s eye, tiger’s eye and hawk’s eye (Cat’s Eye in
Plate-3).
Asterism: In some crystals, particularly those of hexagonal system, inclusions may be arranged
in three crystallographic directions at 120° to each other. A cabochon from such a stone shows
what might be called a triple chatoyancy, that is, one beam of light at each direction of
inclusions producing a six pointed star. This phenomenon, often encountered in star rubies and
sapphires, is termed as asterism (star ruby and star sapphire in Plate-3) and results from
scattering of light from inclusions of rutile arranged in three crystallographic directions. Star
diopside and some phlogopitic mica also show asterism.
3.2.4. Transparency
Transparency affects both beauty and value of a gem. Transparency depends mainly on the
clarity of the substance. Light passing through a mineral is always subjected to –
(a) Reflection of some part at the surface.
(b) Scattering by surface irregularities.
(c) Part absorption if it is colored
(d) Absorption & scattering by inclusions.
14

15. (e) Reflection & scattering by weak planes like cleavage, fracture etc.
Degree of transparency / clarity is very important in gem evaluation. Greater the transparency
higher will be the quality of the gem. Inclusions and other flaws (external or internal) as well as
color reduce transparency. For a light color or colorless stone brilliance depends appreciably on
its transparency. Deep color stones (like ruby, emerald etc.) seldom occur in complete
transparent form. There appears to be an unexplained reverse relationship between the depth of
color and transparency (Flawlessness in rubies and emeralds). Similarly densely included stones
are not very transparent (for this reason chatoyant and star stones are usually not very
transparent). The following is a table of comparative transparency.
Transparent An object viewed through the stone can be seen clearly
(e.g. rock crystal, topaz etc).
Semitranspare
nt
The image of an object viewed through the stone will be
blurred but still recognizable.
Translucent The stone will transmit some light but object cannot be
seen through it (e.g. chrysoprase, jadeite).
Opaque The stone is sufficiently dense optically to prevent the
passage of any light (e.g. malachite, turquoise).
3.2.5. Refractive Index
Refractive index of a crystal is the
Sine of the angle of incidence
R.I. = -------------------------------------
Sine of the angle of refraction
However, only glass and crystals in isometric system are singly refractive while all minerals
belonging to tetragonal, trigonal, hexagonal, orthorhombic, monoclinic and triclinic systems
have two refractive indices.
Refractive index / indices (R.I.) of each mineral is unique. There of course are overlaps in
ranges, but still, determination of R.I. along with other tests can help in mineral identification.
The RI of a gemstone/gem is determined by Refractometer (Plate-2). The instrument is
designed optically to use the phenomenon of critical angle (total internal reflection) to provide
direct RI reading and is also known as critical angle refractometer. However, the principle can
only be used if the RI of the gemstone being tested is less than the refractometer’s lead glass
prism which has an RI of 1.86 (if the gemstone’s RI is greater than the RI of this prism, the ray
will be refracted out and there will be no total internal reflection). The gemstone is placed on
this glass prism in such a way that one of its flat facets is in good contact of the prism (Fig 3.1).
In reality, however, a contact fluid (saturated solution of sulphur in di-iodomethane and
15

16. tetraiodoetylene with RI = 1.81) is used to ensure good optical contact between the gem and the
lead glass prism. The principle of total internal reflection occurs as follows:
(i) As light converge from the prism onto the surface of the gem (Fig 3.1a), ray I1 and I2 (which
have larger angle of incidence compared to the critical angle) are reflected back into the denser
prism following the laws of total internal reflection. Rays I4 and I5, whose angle of incidence is
less than the critical angle are refracted into the gem. But ray I3, which is incident just at the
critical angle travels along the interface of the two mediums. Thus when light rays passes from a
dense medium to a rarer medium of gemstone, the light rays will be reflected back from the
surface of the gemstone over an arc of incident angle greater than that of ‘critical angle’ of
incidence. This ‘critical angle’ is determined by the RIs of both the denser medium and the
gemstone. The dense medium in the refractometer is a glass prism of known RI. The ‘critical
angle’ gives the direct measure RI of gemstone as follows:
RI of rarer medium (gemstone)
Sine of ‘critical angle’= ----------------------------------------
RI of dense medium (prism of refractometer)
RI of gemstone = sine of critical angle x RI of refractometer prism
RI for gemological purpose is defined in terms of yellow monochromatic light having a
wavelength of 589.3 nm (sodium light) which gives sharpest and most easily seen shadow edge.
Fig 3.1a Fig 3.1b
The basic construction of the critical angle refractometer is shown in Fig 3.1b (from Read,
1997). Here, the light rays arriving at the interface between the gemstone and the glass prism
and having an angle of incidence less than the critical angle (ION) are not reflected into the lens
system. However, those rays having an angle of incidence greater than the critical angle are
reflected into the lenses and illuminate a scale graduated in RI values. The image of the scale is
inverted by a mirror and then focused by the eyepiece. The end result is viewed as a dark top
section and an illuminated lower part. The horizontal shadow edge between the two parts is the
measurement of the refractive index of the gem.
3.2.6. Luminescence
16

17. In contrast to the color of a gemstone, which is observed in daylight, there are certain minerals,
which produce color or visible light in darkness under certain special circumstances. This
phenomenon is known as luminescence. It is found that when certain materials acquire surplus
energy in one form or another (but below the level of burning or glow) they convert this energy
into a ‘cold’ radiation whose wavelength generally lies in the visible section of the spectrum.
The mechanism producing this ‘cold radiation’ or ‘luminescence’ is associated with the
excitation of atoms within the material. The surplus energy acquired by luminescent substances
is used up in moving electrons out of their normal orbital state (ground state) temporarily into
orbits of a higher energy level (excited state). This high-energy state is unstable so the electrons
relax into a lower energy excited state that is slightly more stable. When these electrons
eventually return to their more stable orbits (ground state) they give up the surplus energy in the
form of electro-magnetic radiations. This emitted energy is always less than the excited energy.
Since wavelength increases as the energy decreases, emission occurs at larger wavelengths than
the excitation wavelengths. For example, stimulus of shorter wavelengths of ultraviolet (less
than ~ 400nm) rays can result in emission of longer wavelengths in the visible range (e.g.
natural ruby frequently give red [~700nm] luminescence under long wave U.V).
In case of gemstones, the best stimulant is the radiation by invisible shorter wavelengths or
ultraviolet rays (UV). UV lamps that produce light of two different wavelengths check
luminescence of gemstones normally.
1. Short wave UV lamp (253.7nm)
2. Long wave UV lamp(365 nm)
Certain minerals respond better to short wave UV radiation while some fluoresce better in long
wave UV radiation.
A substance is fluorescent if the emission of light stops as soon as the energy source causing it
is removed; if it continues to glow even after the source of stimulant is cut off, then it is called
phosphorescence (e.g. Kunzite). Luminescence (fluorescence / phosphorescence) should
always be checked in dark room.
In all forms of luminescence, the light emitted is either due to some intrinsic property of the
material (e.g. lattice defect in diamond) or due to the presence of luminescent impurities called
activators (e.g. Cr2O3 in ruby).
3.2.7. Pleochroism
As pleochroism is an useful identifying property for a gemstones, an instrument called
Dichroscope is used for quick discrimination. It consists of a cleaved rhomb of optical quality
calcite (Iceland spar), which is mounted, in a glass tube having an eyepiece at one end and a
square aperture at the other end. A glass prism is cemented to each end of the calcite rhomb to
allow the light to enter and leave in a straight line. When the colored gemstone, if doubly
refracting and pleochroic, is viewed in direction other than that of an optic axis, the two images,
17

18. which appear side by side, will differ in shade or color. Singly refracting stones will not show
the change of shade or color.
3.2.8 Optic Sign and other optical properties
Ordinary unpolarized light waves vibrate in all directions at right angles to their line of travel. If
unpolarized light passes through a doubly refractive material (such as gemstone) it emerges as
two separate polarized rays. These rays will vibrate only in a single plane at right angle to each
other and to their direction of travel. Polarizing filters/ Nicol prism helps in producing this
polarized ray by separating out ordinary rays from the extra-ordinary ray.
Polariscope, a gemological instrument, uses two set of polarizing filters for getting crossed or
extinction position. This crossed/extinction position has practical application in identification of
gems.
Use of Polariscope:
1. Optical character: - To ascertain isotropism/anisotropism of a gem mineral. In isotropic
(singly refracting) gems there will be little light visible because the polarized light from
the bottom filter passes through the sample without being changed by it and is then
blocked by the top filter while rotating the top filter. If the whole stone appears to
alternate between transmitted light and then blocking it (appearing alternate light and
dark) during 3600
rotation, the stone is anisotropic (doubly refracting). The test may be
done in more than one direction of the specimen to avoid c-axis position. Red spinel can
be differentiated from a ruby by this method.
2. Strain: - Many gems and synthetics like diamond, synthetic spinel, glass etc show internal
strain which result in anomalous double refraction when viewed in polariscope. This
usually appears as vague dark patch or band moving across the sample as it is rotated.
The similar patchy feature seen in synthetic spinel is called ‘tabby extinction’.
3. Interference figure: - Interference figure (uniaxial or biaxial) of an anisotropic stone can
be produced by holding it under crossed filter and superimposing a spherical bead
(conoscope) over it. The sample has to be rotated till a distinct first order interference
colour is obtained.
4. Pleochroism: - In some doubly refracting colored gemstones the two rays travelling in
different speeds may emerge differing in shades or color. The rays are said to have
experienced differential selective absorption. This is called pleochroism (Dichroic if light
split into two colours/shades-uniaxial, trichroic if light split into three colours/shades –
biaxial).
3.2.9. Spectroscopy
As discussed earlier, the perceived color of most objects is the result of their ability to absorb
certain wavelengths or colors in the light passing through them or reflected off their surface.
This suppression of part of the spectrum in the illuminating light is known as selective
absorption and helps in identification of some gemstones.
In majority of the gemstones the color is related to the presence of transition elements and this
color is due to the selective absorption of wavelengths in the light illuminating the gemstones.
18

19. To know which wavelengths have been absorbed, a ‘spectroscope’ is used. The spectroscope
spreads out the light from the gemstones into spectral colors. Absorbed wavelengths will form
dark lines or bands across the spectrum and is called absorption spectrum. The transition
elements in a gemstone produce these lines, bands or doublets (two closely spaced lines). The
position of these lines or bands can be different for different gems having same transition
element.
In some cases light, which is illuminating the gemstone, stimulate the transition element, instead
of absorption, they emit light at these same wavelengths to produce emission spectrum
(fluorescent lines). Ruby and spinel produce fluorescent lines or emission spectra.
The direct vision wavelength type prism spectroscope uses triple element Amici prism. This
type of instrument disperses the visible spectrum over angle of 7°
(five element prism produces
about 10°
of dispersion). The prism type produces bright spectra but the spectrum it produces is
not evenly spaced out across the range. This is due to the characteristic of the prism which
compresses the red end and increasingly spreads out it towards the violet end. Faint absorption
lines and bands are difficult to observe in the spread out violet/blue end of the spectrum.
Because the refraction in the prism is dependant on the wavelength of the light, the focus of the
spectroscope also needs resetting while viewing different area of the spectrum. These
disadvantages can be sorted out if diffraction grating is used instead of a prism. The spectrum
produced by this type has the wavelengths distributed evenly and in focus across the spectrum.
But the resulting spectrum is not sharp and bright as multiple spectra are produced on each side
of the main one and tend to dilute it.
Plates 11A, 11B, 11C & 11D provide the positions of the absorption lines for a number of
common gemstones.
CHAPTER - IV
Inclusions & Internal features of natural Gemstones
Study of inclusions and internal features of a gemstone is one of the most fascinating subjects of
gemology. Not only do they provide direct evidences to whether the concerned gem is truly of
natural origin but also hint at the process and place of origin of the gem.
The internal features of a gemstone can broadly be divided into: (1) the features typical of a
natural gem & (2) those typical of a synthetic product. The first has been discussed in this
section while an overview of the second has been provided in Chapter 5 under synthetic
gemstones. A gemologist must however be extremely cautious while certifying a gem as
‘natural’ or ‘synthetic’ based on only one of these features. Solid mineral inclusions, which can
be regarded as a positive sign of a gemstone’s natural origin can be introduced in a synthetic by
19

20. adding small crystals during growth of the gemstone in a flux or hydrothermal solution.
Doublets may contain features of both natural and synthetic stones.
The internal features of a gemstone can broadly be divided into (i) growth zones, (ii) twinning
and (iii) inclusions.
(i) Growth lines / zones : Natural gemstones may possess straight growth lines/zones
(Plate-4). Curved growth bands characteristically indicate synthetic gems.
(ii) Twinning : Some gemstones like corundum, quartz and tourmaline are at times
characterized by single or multiple twinning which generally indicates natural origin. However,
twin planes can now be synthetically generated and therefore no more a confirmatory evidence.
(iii) Inclusions : Though the study of inclusions in a gemstone goes back at least to first
century AD, the science could really make any phenomenal progress with discovery of
microscope in the 17th
century. Inclusions has again been subdivided into:
 (a) Protogenetic inclusions : These are inclusions formed before the formation of the
gemstone. These are strictly solid inclusions. They can be heavily etched or corroded, being
formed long before the gemstone, or well formed euhedral, having formed shortly before the
gem. Calcite inclusions in Mogok ruby (Plate-4), zircon in corundum, actinolite in tourmaline or
emerald (Plate-4) or diamond inclusions in diamond are typical examples of protogenic
inclusions.
 (b) Syngenetic inclusions : These inclusions include solids, liquids and gases and
have developed simultaneously with the host gem. They can be mono-phase (solid/liquid), bi-
phase (solid & liquid or liquid & gas bubble) or tri-phase (solid, liquid and gas).
It is at times very difficult to determine whether a mono-phase solid inclusion is syngenetic or
protogenetic. However, they are generally syngenetic if they can be genetically related.
Examples are spinel octahedral in spinel, apatite inclusions in beryl (both are pegmatitic), mica
or sphene in corundum (both are metamorphic).
Fluid inclusions (mono- or bi-phase) are definitely syngenetic and are very good indicators of
natural origin of a gemstone. Primary cavities
generally result where certain areas of the host
have grown more rapidly than others, forming,
and eventually enclosing, voids. These voids can
be vacant (negative crystals) or contain a single
phase liquid, liquid + gas (2-phase), liquid + solid
(2-phase) or liquid + gas + solid (3-phase).
Photomicrographs of a number of bi-phase and
tri-phase fluid inclusions have been shown in
Plate-4 and Plate-4B. However the bi-phase nail
like inclusion consisting of conical tubes capped
with phenakite crystals (Plate-4), if found in
20

21. emerald, indicates that the gem has been synthetically developed through Linde hydrothermal
process.
The process of development of such syngenetic fluid inclusions is as follows:
• (A) Rapid feathery growth is followed by later steady growth.
• (B) Subparallel growth traps fluids.
• ( C) Dislocation etched out during partial dissolution is later covered by new growth.
• (D) Disturbed growth near a fracture in the surface of a growing crystal results in
trapping of primary fluid inclusions.
• (E) Primary fluid inclusions are trapped between or at the centers of growth spirals.
• (F) Enclosure of any foreign object on the surface of a growing crystal may include
some of the growth fluid as well. (After Roedder, 1984)
Negative crystals can often be recognized by identical crystal orientation to the host, along with
their high relief, due to the liquid or gas filling. More substantial evidence is provided by the gas
bubble sometimes trapped within the liquid.
 (c) Epigenetic inclusions : Inclusions that have developed in a gem subsequent to its
formation are called epigenetic inclusion. Rutile silk in corundum (Plate-4), that is produced
through exsolution from the later is considered to be epigenetic, although the time gap between
host crystal formation and its cooling and exsolution may not be great. Also,
feathers/fingerprints may be epigenetic. In course of its evolution through time, the gemstone
bearing rock might have undergone polyphase deformation and the resulting strain leads to
microfractures in the constituent minerals. Permeating fluids (metamorphic/hydrothermal) enters
these cracks and as the mineral undergoes healing, the fluid gradually forms a veil of
disconnected or partly connected veil like structure known as feathers/fingerprints. Such
secondary fluid inclusion planes are quite common in quartz, corundum, peridot, spinel, topaz
etc. The gemologist must however remain very careful when studying such features because
they are also generated in synthetic gems. However, the later, generally have a twisted
appearance which is never the case in a natural crystal.
As mentioned earlier, inclusions in a gem can indicate the petrogenesis and place of formation
for the particular gem. For example, in Mogok ruby of Myanmer, the commonly noted
inclusions are pargasite, calcite, scapolite, yellow titanite, spinel, pyrite, yellow sphalerite and
short stubby rutile silk needles. Such inclusions definitely indicate a metamorphic environment
in a SiO2-deficient condition, possibly a metamorphosed limestone which, in reality, is the setup
for Mogok rubies. 40
Ar–39
Ar dates from single grains of phlogopite syngenetic with the ruby
indicate Miocene age for Mogok rubies in Myanmar (18.7 ± 0.2 to 17.1 ± 0.2 Ma). The
metamorphism and development of ruby in Mogok, along with rubies formed in Jegdalek in
Afghanistan (Oligocene, 24.7 ± 0.3 Ma), Hunza in Pakistan (10.8 ± 0.3 to 5.4 ± 0.3 Ma),
Chumar in Nepal (5.6 ± 0.4 Ma) etc are all formed during orogeny related metamorphism
related to Himalayan uplift.
21

22. Thinking in the reverse order, the inclusions mentioned above along with the features like v-
shaped reentrant angles in silk, dense white clouds of exsolved TiO2, polysynthtic twinning
accompanied by long, slender associated boehmite needles, hexagonal irregular zoning and
color treacles, very few or no liquid inclusions in a ruby indicate its Mogok origin.
CHAPTER - V
Synthetic Gemstones
A synthetic gemstone is an artificially prepared material having similar composition, crystal
structure, physical and optical properties as that of its natural counterpart. There remains only
subtle differences between natural and synthetic materials that are essential for a gemologist to
understand and identify. This chapter has been divided into two sections: the first part deals in
brief with the different processes of production of synthetic gems while the second is an account
of the various common properties that, under microscope, indicates that a gemstone is synthetic.
5.1. Production of synthetic gemstones
Gemstones are being synthesized since the beginning of the last century. It is part of the
research related to crystal growth science. The productions of this sythesised material have
use in industries and jewelry. Some of them find their way into gem trade as a fraudulent
practice creating global scale concern in identification.
The following is a brief account of the different processes used in production of synthetic
gemstones:
5.1.1. Flame Fusion Process (or Verneuil process)
This method envisages formation of crystal from the melt brought about by passage of powdered
raw materials (high purity alumina powder) through a flame zone. Generally oxide gems like
ruby, sapphires, spinels, star corundum etc. are synthesized by this process. Transition elements
are added as coloring agents to create appropriate color similar to natural ones. Till date this
process is carried out mass scale production of corundum and spinel.
Stones grown by this method generally have gas bubbles / gas clouds inclusions, curved growth
lines, occasional unmelted powder etc. which may be encountered as internal features.
5.1.2. Crystal ‘pulling’ process (Czochralski Method)
22

23. This process produces high purity crystals for LASER and Optical industries. In this process a
seed crystal is lowered into the Iridium/platinum crucible containing the molten source material.
The crucible is heated by using radio frequency (RF) induction coil. When the seed crystal
comes in contact with the molten source it is gradually rotated and slowly pulled at a carefully
controlled rate. The source material crystallizes on the seed and grows downwards as it is pulled
out of the melt. The temperature has to be controlled to get the best result.
This technique helps in synthesizing large crystals ruby, YAG, GGG, scheelite, fluorspar,
Lithium Niobate, alexandrite (Crescent Vart / Inamori created Alexandrite), Cat’s eye
Alexandrite.
5.1.3. Flux-melt growth process
The process is solvent based and uses the old method of dissolving gem constituents having a
high melting point in solvent or flux having a much lower melting point.
A heated platinum crucible is used, in which, the gem-forming chemicals (for emerald:
beryllium and alumina oxides with chromic oxide as coloring agent) are dissolved in solvent or
flux of lithium molybdate heated to about 800°
C. Slabs of silica glass are floated on the melt and
the beryllium and aluminum oxides combine with them to form beryl solution. Seed crystals of
natural or synthetic beryl are lowered into the solution in a platinum cage and the temperature of
the crucible is lowered to a preset level. As the beryl solution is supersaturated, crystals of
synthetic emerald precipitate out and grow on the seed. This process is very slow. Accurate
thermal gradient has to be maintained to enable the source material to dissolve at the bottom and
to recrystallize at the top. The source material is replenished regularly.
This process has been refined subsequently to produce emerald (Chatham, Gilson, Lenix types),
ruby (Kashan, Knischka, Ramaura, Duros types), quartz, alexandrite and rare earth garnets
(YAG, GGG).
5.1.4. Zone melting
This method can be used as a process of refining or growing high purity crystals from powder or
partially fused powder. The equipment consists of an RF induction coil, which is traversed along
the length of the source material, melting it. As the coil moves on, the material cools and
crystallizes or recrystallizes (refining). Alternatively, the induction coil can be stationary, and
the source material moved through it.
Seiko synthetic rubies, sapphire and alexandrite produced by a variant of this process are called
‘floating zone melting’. Crystals made from this process are free from inclusion and growth
features.
5.1.5. Hydrothermal Process
Hydrothermal process involves growth of crystal from aqueous solutions of the source material.
The process is based on the fact that water when heated in an autoclave to more than 400°
C
forms superheated water and steam acts as a solvent for many minerals including quartz. The
hydrothermal method synthesis imitates the process by which quartz crystals and other gemstone
in quartz vein and pegmatitic phases form in nature. By using the solubility of source material in
23

24. superheated water it is possible to produce a supersaturated aqueous solution from which gem
materials can be precipitated and grow on suitable seed crystals. It is a very slow process.
Colorless quartz crystals up to 50X150 mm (mainly for use in electronic industries) can be
grown in 3-4 weeks by this method.
Varieties of synthetic emerald (Lechleitner, Linde, Biron etc.) are manufactured by this process.
Hydrothermal synthetic emerald shows feather like inclusions, nail head inclusions and color
zoning. Lechleitner synthetic emeralds reveal rectangular crack markings and parallel fissures.
5.1.6. Skull-crucible process
This process manufactures synthetic cubic zirconia, which is a commonly used stimulant of
diamond. The source ZrO2 powder has a very high melting point, around 2750ºC. It cannot be
melted in any conventional refractory crucibles. So the crystal of this material is produced by
means of a ‘skull’ melting process. Lebedev Physical Institute, Moscow, developed the method.
The skull crucible consists of circular arrangement of water-cooled copper pipes. The zirconia
powder (ZrO2) along with some amount of stabilizer like MnO, CaO and Y2O3 is packed into the
crucible and is heated by using RF induction coil. The function of the stabilizers is to stabilize
the cubic and transparent states of molten zirconium oxide as it solidifies. Addition of
appropriate rare earth and transition element can produce different color varieties of CZ.
5.1.7. Diamond synthesis
The Swedish company ASEA and General Electric (GE) of USA were first to develop
techniques between 1953 and 1955 to produce industrial grade grit-sized diamonds on
commercial scale. The basic technique involved is dissolving of graphite in molten iron, nickel,
manganese or cobalt at high temperature and pressure. Metal acts as catalyst to reduce the
temperature and pressure to convert hexagonal atomic structure of graphite into more tightly
bonded cubic structure of diamond.
In 1970 GE, America produced carat-sized synthetic gem quality diamonds under laboratory
condition. The technique employed a diffusion process in which free carbon atoms were made to
crystallize on synthetic diamond seeds in the cooler section of a molten metal catalyst ‘bath’.
Small diamonds placed in the hot section were source material for free carbon. The production
cost was prohibitive to produce on commercial scale. In 1986, Sumitomo Electric Industries,
Japan started producing carat-sized transparent yellow synthetic diamonds for various industrial
purposes.
In 1987 De Beers started synthesizing large gem-quality diamonds (largest -11 carats). The main
motive was to seek application of these diamonds in high-technology industries. The diamond
crystals were modified octahedrons. They are made by a flux method (Type Ib- contains
nitrogen atom dispersed through out the crystal lattice and is very rare in nature).
24

25. Constant research is going on to produce synthetic diamonds on commercial scale for use in
scientific industries.
Of these above processes, the Verneuil method (oldest one) has no similarity to the natural
processes of crystallization. The stones grown by this method are, for this reason, the simplest to
recognize by their inclusion patterns. The other processes are somewhat distantly similar to
natural processes. Consequently their products are more like natural gemstones, and for
inexperienced observer, are more difficult to identify.
5.2. Identification of a synthetic gem
It has already been mentioned that differences between natural and synthetic gems is subtle. The
following is a brief overview of the different characters that help in identification of a synthetic
stone.
5.2.1. Growth lines and color zoning
In Verneuil corundum and Verneuil red spinels curved growth lines and curved color zonings
(Plate-5) are generally observed. They are due to intermittent fall of droplets of molten alumina
onto the boule’s upper surface. In contrast, growth lines and color bands in natural corundum are
straight and follow the hexagonal pattern of the crystal system (Plate-4). Such curved bands,
along with gas bubbles (Plate-5) are very useful in identification of synthetically made Verneuil
ruby and sapphire.
5.2.2. Twinning
Twinning of some natural gemstones like corundum, quartz and chrysoberyl, when present,
helps in verification of the natural origin of the gem.
5.2.3. Response under UV
Many a synthetic sapphires and synthetic spinels give distinct fluorescence under SW UV. For
blue sapphires and colorless or blue spinels, the fluorescence is chalk white or chalky blue. Dark
orange-red fluorescence sometimes characterizes the synthetic yellow sapphires. Such
fluorescence under SW UV should be treated as indicators of synthetic origin of the stone.
Synthetic gem quality yellow diamonds produced by GE show no reaction to LW UV but
fluoresce and phosphoresce greenish yellow or yellow under SW UV. On uncut crystals an
occasional large trigon may be present but not in the numbers seen to natural crystals. Similarly,
Sumito gem quality synthetic diamonds are inert to LW UV and fluoresce under SW UV (but
with no phosphoresce). De Beers gem quality diamond synthetics are inert to LW UV, but not
all stones fluoresce under SW UV. Greenish-yellow stones phosphoresce after SW UV
exposure.
5.2.4. Inclusions
25

26. A study of inclusions is the most effective means of identifying a synthetic stone. Natural
inclusions have already been discussed in the foregoing chapter. This section therefore treats
with the inclusions that are likely to be encountered in a synthetic stone.
The synthetic stones frequently contain gas bubbles which appear rounded to elongated (Plate-5)
with thick dark outlines. Such gas bubbles (single/trail or cloud, Plate-5) are definite features of
synthetic origin for a gem. In ruby, often residual flux is left within the stone and it takes on a
dark wispy appearance (Plate-5). Remnant of colored dye is at times encountered in some
synthetic rubies (Plate-5). Twisted feather is another indicator of synthetic origin (Plate-5). In
contrast the natural stones have bi- or tri-phase inclusions or even mono-phase inclusions that
have a distinct look (Plate-4). It should however be remembered that a mono-phase solid
inclusion is not a definite proof of natural origin since such small solid crystals can be added to
the flux or hydrothermal solution during growth of a synthetic. The following are the
characteristic features commonly experienced while studying the synthetic equivalents of some
common gemstones:
(i) Synthetic alexandrite is often characterized by swarms of dust like inclusions and
triangular platinum crystals obtained from walls of the boule. Tadpole shaped gas
bubbles (Plate-5) may also be observed.
(ii) Synthetic emeralds grown from flux melts generally have slightly lower SR and RI
compared to natural ones (SG = 2.65 and R.I. = 1.560, 1.563 in synthetic and SG =
2.71 and R.I. = 1.577, 1.583 in natural crystals). The low value in synthetic stones is
due to absence of Fe which causes high corrosion of platinum of the apparatus.
Twisted wispy veils or curved lace like feathers are occasionally present. Sometimes,
nail like inclusions consisting of fluid tubes capped by phenakite crystals indicate
synthetic origin for an emerald.
(iii) Traces of colorless seed plates and strong color banding parallel to these plates help in
identification of synthetically grown amethyst.
(iv) Curved growth lines or color bands, clouds of minute gas bubbles, tadpole shaped
bubbles (Plate-5), fire marks near facet junctions (parallel cracks caused by
overheating), paint splash, flux filled whitish zones and black distorted hexagonal
platinum and silver platelets are some indicator features for synthetic rubies and
sapphires studied under microscope.
(v) Synthetic spinels, in addition to fluorescence under SW UV, shoe a peculiar cross
hatch pattern (tabby extinction) under crosses polars.
5.2.5. The Plato test for Verneuil corundum
Dr. W. Plato developed this test for identification of Veneuil corundum lacking detectable
inclusions, growth lines and color zonings. First the direction of the stone’s optic axis is found
by using the conoscope adaptation of the polariscope and marked with a felt-tip pen. The stone
26

27. is then viewed in this orientation under crossed polars while in an immersion liquid. If two sets
of bands intersecting at 60° are visible, then the stone is a synthetic Verneuil corundum.
CHAPTER – VI
Gemstone Enhancement
Enhancement of Gemstone to color an otherwise colorless gem, to impart better color to poorly
colored ones or to change color into an attractive shade is an age old practice. As early as first
century AD, Pliny the Second published his 37 volume Natural History in which were
documented many gemstone treatments. Many centuries later, when Camillus Leonardus
published his Speculum Lapidum, he expanded on the earlier writings of Pliny. It appears that
presence of bubbles in glass imitations and even doublets were known to both these writers.
6.1. Foils, Color backing and dyeing
The use of colored foil or paper behind a poorly colored or colorless natural gem in a closed
setting was commonplace in antique jewelry. Mirror backs were also employed to lighten a dark
stone. Another relatively simple way of improving or changing a gemstone’s color was to dye it.
Original organic dyes have now been replaced with inorganic stable ones. Microcrystalline and
polycrystalline gem materials having more porous surface are suitable for this kind of treatment.
For example, a black onyx stimulant has been produced by boiling chalcedony in sugar solution
and then treating the stone with sulphuric acid. It is generally possible to distinguish staining
under microscope specially when there are minute surface cracks that reveal a concentration of
dye. Painting the pavilion of a gem has also been used to enhance its color. With yellow Cape
series diamonds, a thin translucent coat of blue or violet paint on the pavilion will make the
stone appear less yellow.
Most dyes can be removed by washing the stone in suitable solvents. The bluish fluoride coating
(as used on camera lenses) is however more tenacious and requires abrasive and boiling water to
wear off.
6.2. Impregnation of coloring agent
Some gem materials (such as turquoise) are legitimately impregnated with colorless paraffin
wax (or plastic nowadays) to stabilize them and prevent attack from acidic perspiration. Less
legitimate but widely practiced is the use of colored impregnants to increase the value of
colorless or pale colored gemstone. Colorless oils are used to hide surface cracks while colored
oil serves the double purpose of hiding surface flaws and improving the color appearance of
emeralds, rubies, sapphires, opals and ambers. The oiling of stones, especially rubies, is
frequently carried out by the dealers in the mining areas, and it is not unusual to find bottles of
‘red ruby oil’ with the ruby traders in Thailand. Nowadays many synthetic gems are also treated
in this way. The fluid entering the microcracks often resemble ‘feathers’ and render the
synthetic a false appearance of being natural.
27

28. 6.3. Heat treatments
An increasing number of gemstones are now subjected to various forms of heat treatment to
improve or change their color. Many citrines are, for example, the result of heat treatment of
poor-colored amethyst to 450°C. Blue-green aquamarine is heated to around 450°C to get the
popular shades of blue aquamarine. Orange- and apricot coloured beryl when heated to 400°C
produce the pink morganite variety. Pink topaz can be produced by heating yellow or yellow-
brown topaz to 550°C and then cooling. The blue color of sapphire can be deepened by heating
it around 1600°C in a reducing environment. Heating the stone in a similar range in oxidizing
environment causes lightening of color. Stones subjected to heat treatment can sometimes be
identified by the effects of high temperature on inclusions which have a different coefficient of
expansion compared to the stone. They may have expanded producing circular stress features.
Other tell tale signs are total absence of silk, dispersion of the hexagonal color bands and a chalk
white fluorescence under SW UV.
6.4. Surface diffusion
Pale or colorless corundum can be transformed into the rich colors of ruby or sapphire by first
packing the faceted stone into a clay mixture containing the appropriate transition element
(Cr2O3 for ruby and FeO and TiO2 for sapphire). The stones are then heated at around 1750°C
for a period of several days to induce the color producing elements into their surface. In earlier
days the results were at best skin deep (less than a tenth of a millimeter), but thicker layers of
diffusion can now be produced.
6.5. Irradiation methods
The mechanism by which irradiation increases or modifies color in gemstones is to do with the
production of color centers. γ rays, high energy electrons or neutrons are commonly used for
such purposes. Blue topaz is frequently produced from colorless varieties by irradiation and
subsequent heating. Diamond through neutron bombardment can be made to change its color to
homogeneous green which on subsequent heating becomes yellow or golden yellow. When
bombarded with high energy electrons in an electron accelerator, diamond can become pale
blue. However, an ‘open umbrella’ effect around the culet is seen in diamonds irradiated through
the pavilion. For diamonds irradiated through the table, a dark ring will be visible around the
girdle. Also irradiated diamonds have absorption bands in the infrared at 1936 nm and 2024 nm.
6.6. Glass filling
Surface cavities and fractures of faceted rubies and sapphires are at times filled with fused glass
rendering them less visible and improving the overall appearance of the stone. Frequent
presence of bubbles in these fused infillings and their lower r.i. (being more evident if the stone
is dipped in methylene iodide) help in identification of such treatments.
28

29. 6.7. Laser drilling of diamond
Laser drilling is at times used to make a fine hole in a diamond and reach an inclusion. In case
of a dark inclusion, a bleaching agent is leached through the hole onto the inclusion. The hole is
then filled up with transparent gel or epoxy resin.
CHAPTER - VII
Popular Gemstones, their synthetics and simulants
It has already been discussed in Chapter-I that vast majority of gemstones are minerals (mostly
single crystals and in some cases crystal aggregates or cryptocrystalline materials) and are
therefore necessarily inorganic in character. However, a few (like lapis lazuli) are aggregates of
more than one mineral and are thereby rocks. Some other gem materials have organic source.
This section deals with some important and popular varieties of gemstones belonging to each of
these categories. For properties of most other varieties of naturally occurring gems, the reader
can consult Appendix-IV which lists the different gemstones in alphabetical order (prepared by
CGL, Kolkata).
7.1. Gemstones as minerals
Most gemstones are single crystals while some are crystal aggregates. Yet others can be
cryptocrystalline materials. In this section, a few precious and semi-precious gemstones that
have been traditionally valued in India have been discussed. The other gemstones have been
listed in alphabetical order in Appendix-IV (prepared by CGL, Kolkata).
7.1.1. Diamond
Diamond, the king of gems was traditionally known as ‘vajra’ in ancient India and ‘Heera’ in
present days. Compositionally, diamond is almost pure carbon with covalent bonds forming
mostly octahedral (Plate-3B), but also cubic or dodecahedral crystals in isometric system.
Twinned octahedra are called ‘macles’ and have re-entrant angles.
Diamonds are highly prized if they are colorless and transparent. But it may also occur in shades
of yellow (Cape series), brown (brown series) and green (Plate-3B). Also ‘fancy’ shades of
diamond like red (very rare), pink, orange, yellow, brown, blue (e.g. the famous Hope diamond),
green are also available. Industrial diamonds are generally of poor color and quality and are
often microcrystalline (boart). Gem quality diamonds have an adamantine luster.
29

30. Diamonds have the highest hardness of all known minerals, 10 in Moh’s scale. The cleavage is
perfect, octahedral and plays a vital role during gem cutting. Also such perfect cleavage makes
the gem brittle in spite of having such great hardness. Fracture is conchoidal to irregular.
Specific gravity of diamond is 3.52. Refractive index is 2.417. Dispersion is high (0.044).
Depending upon their fluorescence and energy absorption, Robertson, Fax & Martin (1934)
classified diamonds broadly into two types. They noticed that some diamonds are transparent to
300 nm UV as also to 8μm infrared radiations. All other diamonds are opaque to both these
wavelengths. They named the common opaque type as Type I diamond and the transparent type
as Type II diamonds. Type I diamonds contain nitrogen as an impurity. In Type Ia the nitrogen
atoms occur in clusters which do not affect the stone’s color. In Type Ib the nitrogen atoms are
dispersed throughout the crystal lattice causing the yellow color of Cape Series diamonds.
Natural diamonds are mostly a mixture of Type Ia and Type Ib. Type I diamonds fluorescence in
various colors under LW UV.
Type Ia diamonds can further be subdivided into Type IaA (which contain nitrogen atoms in
pairs) and Type IaB (which contain groups of three nitrogen atoms the N3 centres – and/or larger
groups or aggregates of even numbers of nitrogen atoms (four or six)
Type II diamonds contain no nitrogen impurities. Type IIa diamonds are pure carbon and
contain no impurities. They do not phosphoresce when irradiated with LW or SW UV light.
Type IIb contain boron impurities and irradiated with UV radiation produces a bluish
fluorescence. Sometimes there is a bluish or even red afterglow (e.g. the famous Hope
Diamond).
Type III diamonds were discovered in meteorites by Dame Kathleen Lonsdale (and named
‘Lonsdaleite’). They have a hexagonal instead of cubic crystal structure.
Colourless to yellow Cape series stones have an absorption band in the violet at 415.5 nm (plus
other weak bands in the violet in strongly coloured stones). Brown series stones have a band in
the green at 504 nm sometimes with two weaker bands in this area.
Diamonds have high thermal conductivity which differentiates it with most diamond stimulant
except for moissanite. At room temperature, the thermal conductivity of Type I diamond is
about 1000 watts/m/°C and Type IIa is 2600 watts/m/°C. In contrast, the most common diamond
stimulant CZ (Cubic Zirconia) has a conductivity of only 10 watts/m/°C and white sapphire
(another stimulant) has 40 watts/m/°C. This difference in conductivity can be used for
identification of diamond. Conventional diamond testers (thermolysers, Plate-2) differentiate
between diamonds and most simulants on the basis of this difference in thermal conductivity.
Normally diamond is a good insulator. However, Type IIb diamonds have lattice imperfections
with loose electrons. A potential difference applied to such a diamond will cause a migration of
electrons to the positive pole. This flow of electrons is small at first, only a few milliamperes,
but the flow tends to heat up the crystal which liberates more and more electrons from the atoms
and the current increase rapidly to several amperes.
30

31. Crystal inclusions in diamond include hematite, diamond, diopside, enstatite, garnet, olivine,
zircon, black inclusions may be iron ores or graphite.
Diamond can be enhanced. Diamonds irradiated through neutron bombardment can be made to
change its color to homogeneous green which on subsequent heating becomes yellow or golden
yellow. When bombarded with high energy electrons in an electron accelerator, diamond can
become pale blue. All natural blue diamonds are Type IIB and therefore electroconductive
whereas blue diamonds created through enhancement are nonconductive. Therefore, this
property is frequently used to separate natural blue diamond from enhanced ones. Also, an ‘open
umbrella’ effect (Plate-3B) around the culet is seen in diamonds irradiated through the pavilion.
For diamonds irradiated through the table, a dark ring will be visible around the girdle. Also
irradiated diamonds have absorption bands in the infrared at 1936 nm and 2024 nm. Laser
drilling is at times used to make fine holes in a diamond and reach the dark inclusions (Plate-
3B). A bleaching agent is then leached through the hole onto the inclusion. The hole is then
filled up with transparent gel or epoxy resin. The yellowness of the Cape series Type Ib
diamond is attributed to dispersed nitrogen throughout the lattice. By heating these diamonds
under very high temperature and pressure conditions available in the diamond synthesis plant,
the nitrogen atoms can be made to group into clusters thereby converting the diamond to Type Ia
which is colorless.
Diamond has also presently been synthesized artificially as has already been discussed in
Chapter-V.
However, the most pressing problem in diamond market is not the synthetics which are still too
costly for mass production, but the diamond simulants (both natural and synthetic). Table 7.1
gives a list of commonly used diamond simulants and their constants that can aid a gemologist
in detecting the actual diamond.
Table 7.1. A list of diamond simulants and their physical and optical properties
Gemstone R.I. DR Dispersion SG H
Diamond 2.417 - 0.044 3.52 10
Flint glass 1.6-1.7 - 0.04 3.0-4.0 5
Quartz 1.54-1.55 0.009 0.013 2.65 7
Topaz 1.61-1.62 0.01 0.014 3.56 8
Zircon 1.93-1.99 0.058 0.039 4.68 7
Synthetic spinel 1.727 - 0.02 3.64 8
Corundum 1.76-1.77 0.008 0.018 3.99 9
Synthetic rutile 2.61-
2.897
0.287 0.280 4.2-4.3 6.5
YAG (Yttrium
aluminium garnet)
1.83 - 0.028 4.58 8.5
Strontium titanate 2.41 - 0.190 5.13 5.5
Lithium niobate 2.21-2.30 0.09 0.120 4.64 5.5
GGG(Gadolinium
gallium garnet)
1.97 - 0.045 7.05 6
31

32. (CZ) Cubic zirconia 2.15-2.18 - 0.065 5.6-6.0 8
Moissanite 2.65-2.69 0.104 3.21 9.25
7.1.2. Corundum
The corundum group (Al2O3) of gemstones crystallizes in the hexagonal system. The famous
gemstones belonging to this group are ruby and sapphire. Ruby (known as ‘Manek’ or
‘Manikya’ in ancient India) occurs as hexagonal prisms (Plate-6) and varies in color from dark
red to pigeon blood red to deep pink. The name ruby itse;f has been derived from the Latin worb
‘ruber’ or ‘rubrum’ meaning red. Sapphire (‘Neelam’) has a bipyramidal habit and varies widely
in color (Plate-6). Accordingly they are termed blue sapphire, pink sapphire, yellow sapphire
(Pushyaraga or kanakapushyaraga), violet sapphire, orange-yellow sqapphire (Padparadscha),
white sapphire (colorless variety) and even green sapphire. However, originally the term
sapphire was reserved for the blue variety, the name being derived from the Greek work
‘Sapphirus’ for blue.
A variety of transition ions, when present in trace amounts replacing for Al, are responsible for
the wide range of colors encountered in the corundum group of minerals. Table 7.2 list the
causative ions responsible for coloration in different varieties of rubies and sapphires.
Table 7.1. Allochemical chromophores in different gem varieties of corundum.
Cr+3
Red
Cr+3
+ Fe+2
Dark red or brownish red
Fe+2
Pale green
Fe+3
Yellow (through Fe+2
-Fe+3
or Fe+2
-O transfer
Fe+3
+ Ti+4
Colorless or pale yellow
Fe+2
+ Ti+4
Blue (through Fe+3
+ Ti+4
charge transfer)
Fe+3
+ Ti+4
+ Fe+3
Green
Fe+3
+ Ti+4
+ Cr+3
Violet
Fe+3
+ Cr+3
+ Cr+4
Orange - Padparadscha
V+3
+ Cr+3
Greenish blue to bluish violet
The specific gravity of corundum is 3.99 to 4.1, hardness 9 in Moh’s scale and refractive index
1.76-1.77. DR is 0.008, dispersion low (0.018) and optic sign uniaxial negative. Cleavage is
poor, fracture conchoidal to irregular and luster vitreous to subadamantine. Pleochroism is
strong in ruby (deep red, orange red) and medium in blue (blue, green blue); yellow and green
sapphire (two shades of body colour) orange sapphire (orange, colourless), purple sapphire
(violet and orange).
Corundum generally shows lumiscence under UV lamp. Table 7.3, prepared by CGL, Kolkata,
summarizes the fluorescence encountered in the different varieties.
Table 7.3. Fluorescence in corundum
32

33. Gemstone LWUV SWUV X-rays
Ruby (natural and
synthetic)
Red Red Red (synthetic shows
phosphorence)
Sapphire (Pink, natural and
synthetic)
Red Red Red
Sapphire (Green synthetic) Red Inert Inert
Sapphire (Orange synthetic) Red Red Red
Sapphire (white) Orange Inert Orange
Sapphire (Yellow- Sri
Lanka)
Aprocot Apricot Apricot
Sapphire (Blue-Sri Lanka) Inert Pink Pink
Sapphire (Blue-Synthetic) Inert Green/blue
(some)
Green/blue (some)
Absorption spectrum for ruby consists of a doublet plus two further lines in the red, broad
absorption band centered on 550 nm. With appropriate lighting the doublet lines in red may be
seen as emission rather than absorption lines. For sapphire there is an indication of iron
absorption band at 450 nm (in iron rich stones-471, 460 and 450 nm).
Under microscope, straight hexagonal zoning is a common feature in natural ruby. Twinning is
also encountered at times. The inclusions can be mono-phase solid or feathers. Inclusions in
corundum are of special significance. As already mentioned in Chapter-4, these inclusions speak
a lot regarding the genesis and locality of formation of the gemstone. The following is a list of
inclusions commonly encountered in rubies from different areas:
• Myanmar (Burma): Zircon, spinel and rounded colourless crystals, rhombs of calcite and
yellowish sphene crystals. Rutile needles (Silk), wisp and swirls of colour (Treacle).
• Sri Lanka: Long sparse rutile needles (Silk), zircon crystal with ‘haloes’, pyrite and
biotite mica.
• Tanzania: Rutile needles, apatite, zircon and calcite crystals. Whitish boehmite particles
along intersecting twiniing lamellae planes.
• Thailand: Fewer inclusions than Myanmar stones. Reddish-brown opaque almandine
crystals. Partly healed cracks or feather surrounding crystals, yellowish apatite platelets.
Very little silk (Rutile needles).
The inclusions in sapphires are
• Australia: Strong colour zoning, zircon crystals with haloes as in Sri Lanka sapphires,
crystals of plagioclase feldspar.
• Cambodia: Plagioclase feldspar and red pyrochlore.
• India (Kashmir): Milky zoning or cloudiness caused by layers of liquid incusions;
feathers and zircon crystals with haloes.
• Myanmar: Convoluted healing feathers (looking like crumbled flags), short thick rutile
needles, apatite crystals.
33

34. • Sri Lanka: Rutile needles forming silk, three phase inclusions, zircon crystal with haloes
and feathers, lines of spinel octahedral. Elongate negative crystals.
Star corundum is a special variety of ruby or sapphire with retile exsolutions along the
crystallographic planes occurring in such a way that a properly cut cabochon will reflect a six
rayed star on the C-axis. A twelve rayed star is occasionally visible due to twinning of the
corundum.
Gem quality ruby comes from Afghanistan, Myanmar(Burma), Cambodia, Pakistan, Sri Lanka,
Tanzania, Thailand and India while sapphire is mined in East Africa, Kampuchea, Kashmir,
Thailand, Australia, USA. In India, corundum occurs in Andhra Pradesh (poor quality ruby and
sapphire and star corundum), Jammu and Kashmir (blue sapphire), Karnataka, Kerala, Orissa,
Tamil Nadu, Meghalaya and Madhya Pradesh. For further detail, the reader is referred to the
special volume ‘Gems and Gem Industry in India’ by R. V. Karanth.
The simulants of ruby include pink tourmaline or rubellite, red garnet (sold as Arizona Ruby or
Cape Ruby), garnet or rose quartz (sold as American ruby), red spinel and pink topaz. Also red
glass and paste are frequently marketed as ruby. Plate-6 shows a spinel (a part of the British
crown jewelry that was originally taken to be a ruby). The common stimulants of blue sapphire
are kyanite, synthetically prepared blue spinel, blue tourmaline or indicolite, iolite and zoisite
along with glass imitations. Yellow sapphire is often simulated by citrine (generally burnt
amethyst) and even oligoclase.
Enhancement, as discussed in Chapter-6 is also common in the corundum group of gems.
Synthetics are very common for the corundum group. However, features like curved growth
lines (Plate-5), remnants of dye (Plate-5), bubbles (Plate-5) and twisted feathers (Plate-5) can
give them away. The gemologist must also be careful about doublets (Plate-5) which may bear
inclusions of both natural and synthetic origin.
7.1.3. Beryl
The beryl group of minerals (Be2Al2 (SiO3)6) occur as hexagonal prisms and have a wide variety
of color and types : emerald (green, Plate-7, known as Panna in ancient India), aquamarine (blue
green, Plate-7), heliodor (golden/yellow, Plate-7), morganite (pink, Plate-7), goshenite
(colorless), bixbite (red) and maxixe-type (dark blue which fades in daylight).
Depending upon substitutions, the specific gravity of beryl varies from 2.7-2.8, hardness is 7.5-
8.0, refractive index 1.56-1.59, DR 0.005-0.008, low dispersion (0.014) and uniaxial negative
optic sign. The luster is vitreous. Poor basal cleavage is generally present.
Pleochroism of beryl is medium to weak (emerald, morganite, bixbite, vanadium beryl – shades
of body colour : aquamarine – body colour and near colourless ; maxixe and maxixe – type
show light –blue dichroism when viewed perpendicular to optic axis (aquamarine shows darker
34

35. dichroic colour in the orientation). Luminescence in emerald is medium red to pink fluorescence
under LW UV, SW UV and X-rays, but inhibited by iron oxide content, morganite – crimson
fluorescence under X-rays. The absorption spectrum of emerald shows significant differences in
spectrum between the ordinary and the extraordinary ray. The ordinary ray has a doublet in the
deep red (680/683nm), a line at 637 nm, a broad weak absorption band centered on 600 nm in
the yellow, and in chrome-rich stones a line in the blue at 471 nm. In the extra ordinary ray, the
doublet is stronger but the 637 nm line is missing; in its place are two diffuse lines at 646 and
662 nm, the broad absorption band is much weaker and there are no lines in the blue. Blue
aquamarine has weak bands in the blue (456 nm) and violet (427 nm). Green aquamarine has a
537 nm band in the green in the extraordinary ray. Maxixe and maxixe-type dark-blue beryls
have bands in the red (695, 654 nm) with weaker bands in the orange, yellow and yellow –green
(628, 615, 581, 550 nm)
As in corundum, the inclusions in beryl are good indicators of the place of origin. The following
is a list of inclusions observed in emeralds from different places:
• Brazil: Biotite mica and thin liquid films resembling paving stone.
• Columbia: Three phase inclusion with jagged ends, albite and pyrite crystals (Chivor
mines); rhombs of calcite and yellow/brown rhombs of parasite (Muzo mines).
• India: Hexagonal negative crystals comprising two phase inclusions resembling
‘commas’ and mica.
• Pakistan: Flakes of mica, crystals of phenkite and thin liquid films resembling the veil
type of inclusions in flux-melt synthetics.
• South Africa (Transvaal): Green mica flakes of fuchsite.
• USSR (Siberian): Flakes of mica and green actinolite crystals in blade form.
• Zambia: Tourmaline crystals, mica flakes, elongate two phase inclusions, fibrous and
acicular crystals.
• Zimbabwe (Sandawana): Hair like tremolite fibres, mica.
Aquamarine – two-phase inclusions, ‘rain’ and mica (most stones are free of inclusions).
The simulants after emerald include demantoid garnet, green sapphire, peridot, jadeite, green
tourmaline (verdite) in addition to imitation glass and paste. Synthetic spinel, topaz, zircon and
glass are the aquamarine simulants. However, the SG and RI of the simulants give them away.
Synthetics are petty common. The gemologists should be cautious of features like bubbles,
twisted wisp-like feathers, nail-head inclusions, phenakite crystal inclusions and a network of
fine surface cracks that commonly indicate that the emerald is synthetic. Also most flux melt
synthetic emeralds have significantly lower RI (1.560-1.563) compared to their natural than
natural emeralds.
7.1.4. Garnet
Garnet is an isometric aluminosilicate normally developing a dodecahedral crystal form,
icositetrahedral form being much less common. The principle end member varieties of garnet
35

36. are pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), spessertine (Mn3Al2Si3O12), grossular
(Ca3Al2Si3O12), andradite (Ca3Fe2Si3O12) and uvarovite (Ca3Cr2Si3O12). However, extensive solid
solutions are possible between these end members. Some additional terms for gem variety
garnets are:
• Hessonite : Grossular garnet
• Demantoid : Andradite garnet
Table lists the properties of the different end member garnets.
Table : Physical and optical properties for the end member garnet compositions.
Variety Refractive index Dispersion Specific Gravity Hardness
Almandine 1.76 – 1.81 0.024 3.8 – 4.2 7.5
Andradite 1.89 0.057 3.85 6.5
Grossular 1.74 – 1.75 0.028 3.6 - 3.7 7.0 – 7.5
Pyrope 1.74 -1.76 0.022 3.65 – 3.8 7.25
Spessartine 1.80 -1.82 0.027 4.16-4.19 7.25
Uvarovite 1.87 0.030 3.77 7.5
Gem variety pyrope is red in color and has a vitreous luster. They are seldom found in large
sizes. Also their color, though fine, is frequently too dark to be appreciated. Pyrope can be
confused with red spinel but the constants are different. The absorption spectrum of pyrope has a
dark band at 575 nm (yellow-green).
Almandine is red in color (sometimes purplish red) and has a vitreous luster. Sometimes the
color is so deep that it appears almost black. Siam ruby has almost the same color but can
readily be distinguished by SG and R.I. Also ruby is doubly refracting whereas isometric
almandine has single refraction. Spectroscope provides an yet more certain test (the absorption
lines of both ruby and almandine has been provided in Appendix-V). Almandine doublets are
sometimes encountered with a table facet of almandine and a pavilion of glass. In such cases
there will be a layer mineral inclusions followed by typical gas bubbles. At times, the inclusions
in almandine, oriented parallel to the dodecahedral faces can produce a weak star effect.
Spessertine can range in color from yellow through orange to flame red. Often it closely
resembles hessonite garnet in appearance though it lacks the granular inclusions and treacly
swirls common in hessonite. Solid solution with almandine renders wine color to the gem. The
constants are close to almandine but the absorption spectrum can be useful in distinction
(Appendix-V).
Grossular (hessonite) occurs in shades of orange-brown or honey-brown. Under microscope it
has a peculiar granular appearance resulting from numerous small inclusions.
A massive green variety of hydrogrossular has been found in South Africa and misleadingly
called Transvaal jade. Also pink to rose red varieties of this hydrogrossulars are used as gems.
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37. They have an SG range of 3.28-3.57 and sometimes even 3.63. The R.I. for the later is 1.738.
Hydrogrossular gives orange fluorescence under X-rays.
Andradite is a green variety of garnet and is called demantoid in its gem variety. A peculiarity of
most of the demantoids is the nature of its inclusions. These are tiny silky asbestos fibers often
radiating from one or more centers. Also demantoid can have a vitreous to subadamantine luster
and might appear pink through Chelsea filter.
Uvarovite is a green garnet but rarely occurs as gem.
The inclusions present in almandine garnet are the acicular crystals of rutile. Demantoid has
typical ‘horsetail’ inclusion consisting of radiating byssolite (asbestos) fibres. Grossular
(particularly hessonite variety) contains profusion of small crystals of apatite and /or zircon
giving a treacly (oily treacle) appearance while pyrope has occasional needle-like crystals.
Spessartite – Shredded-looking feathers are sometimes present in spessertine.
7.1.5. Tourmaline
Tourmaline is a complex borosilicate having a wide range of substitutions. It occurs widely but
is rarely of attractive color and transparency to be used as a gemstone. Tourmaline crystallizes in
the trigonal system. It has a prismatic habit with a pyramidal apex and pedial base. Vertical
striations on the prism faces result from alternate twinning.
The alkali-tourmalines (containing Na and at times Li) are the most important from the point of
view of gemology. These have the lowest SG (varying between 3.01-3.06). Their color varies
from attractive pink or red (rubellite, Plate-8), green (verdite, Plate-8) or colorless (achroite,
Plate-8). The magnesian tourmalines (dravite) are brownish and have SG = 3.04-3.10. The iron
rich tourmalines (schrol) vary in color from deep greenish blue to black (SG = 3.08-3.20). Blue
tourmaline is known as indicolite (Plate-8) and is highly prized. The best stones are pure blue
without hints of green or gray. It tends to be of similar color to dark blue topaz. A new variety of
light and very lively blue was discovered in Parabia, Brazil, and has achieved the highest prices
paid for tourmaline. Analysis of this material show trace amounts of gold in the structure. Blue
stones can be found that are large and flawless. They do not suffer from the poor structure found
in rubellite. Yellow and orange tourmaline (Plate-8) maintains intermediate value as long as it
does not move into the brown region. Clean yellow and bright orange stones are sought after by
collectors and find their way into a small amount of commercial jewelry. Brown and orange-
brown stones are quite common and are not highly valued. There is a special variety of
tourmaline that shows a pink/red core and a green rim along the length of the prism. It is called
"watermelon" tourmaline (Plate-8), and is often cut and polished flat across a crystal face. It is
sometimes even faceted. Due to compositional gradations through substitution, color variations
are at times observed along the length of a single crystal (Plate-8). The refractive index of
tourmaline is in the range 1.62-1.64. When the stone is rotated the lower index moves while the
higher one remains constant. Also tourmaline can show weak to quite strong pleochroism
depending upon the depth of body color. It is weakest in the pale green stone. Tourmaline is
uniaxial negative. Blue and green stones have a strong absorption band in the green at 498 nm,
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