Gemology/Gemmology in forensic science .pptx

Gemmology
BHANU PRATAP SINGH YADAV
CRIME SCENE MANAGEMENT &
PHYSICAL EVIDENCES
NFSU– DELHI CAMPUS

Contents:
Introduction to Gemmology: Origin, Development and Scope
Gemstone Examination:
Physical Methods
Optical Methods
Fluorescence Techniques
Spectroscopic Techniques
X-Ray Diffraction Studies

Introduction to
Gemmology The scope of gemmology encompasses a multifaceted range of activities, including
the identification and classification of gemstones based on their distinct properties,
the exploration of geological processes influencing their formation, and the
valuation of gemstones considering factors such as rarity and clarity.
Gemmologists play a crucial role in detecting treatments or enhancements applied
to gemstones and collaborate with jewelers in crafting aesthetically pleasing
jewelry designs. Moreover, staying informed about market trends is essential in
navigating the dynamic gem industry.
From a forensic standpoint, gemmology assumes a pivotal role in authenticating
gemstones, detecting fraud or substitution, and providing expertise in crime scene
analysis. The ability to trace the origin and history of gemstones contributes to
establishing ownership and uncovering potential involvement in criminal activities.
Furthermore, gemmologists engaged in forensics scrutinize documentation, such
as certificates and receipts, to verify authenticity and offer expert testimony in
legal proceedings related to gemstone crimes.
In essence, the forensic scope of gemmology ensures precision and accountability
in the examination and analysis of gem-related evidence within a legal context.
What is Gemmology?
Gemmology is the science of
studying natural and artificial
gemstones, their properties,
characteristics, identification,
grading, and valuation.
Gemmologists are experts who
can examine, appraise, and
certify gemstones and
jewelry. Gemmology is a branch
of mineralogy and geology, and
it requires specialized
knowledge and equipment.
20XX 3

Introduction to
Gemmology
Origin of Gemmology
Gemmology, the science dedicated
to the study of gemstones and their
technical aspects for identification
purposes, has a rich history spanning
over 2000 years. While early
observations date back to
philosophers and scientists
captivated by the beauty of gems, it
wasn't until the latter half of the 19th
century that gemmology emerged as
a specialized branch, branching off
from mineralogy. The science gained
significance in identifying modern
synthetic gems, leading us to the
exploration of its historical
milestones.
2023
Ancient
history
• Over 2000 years ago, Philosophers & astronomers showed great interest
in Gems.
1652
• Thomas Nichols publishes one of the first books on Gems in English.
1837
• Marc Gaudin synthesizes small Ruby crystals.
1866
• Sir Arthur Church discovered absorption bands in spectrum of Zircons
and Garnets
1873
• Kimberley Mining Town Established in South Africa; Diamond Rush is
made official.
1902
• Discovery of Pink Kunzite.

20XX PRESENTATION TITLE 5
1913
• Gemmological Association of Great Britain is formed.
1925
• Basil Anderson sets up a pearl testing lab in London.
1928
• 1st Synthetic “Spinel” produced by Verneuil Method.
1931
• Robert Shipley establishes Gemological Institute of America (GIA).
1935
• GIA launches “Gems & Gemology” Journal.
1940s
• Intensive research on Gemstone identification during WW-II
1951
• New gemstone species “Taaffeite” discovered.
1959
• Reflectance instrument for gemstone identification developed.
1963
• J. Lechleitner introduces “Synthetic Emerald”.
Taaffeite

20XX 6
1967
• Discovery of “Tanzanite” in Tanzania.
1971
• Russian scientists claim to synthesize diamonds.
1974
• Dr. Eduard Gübelin publishes “Internal World of Gemstones”.
1975
• Reflectance Meters for Gemstone identification introduced.
1986
• Sumitomo Electric Industries produces gem-quality synthetic diamonds.
1990
• The Gemmological Association of Great Britain merged with London
Gem Testing Lab.
1993
• Yellow gem-quality synthetic diamonds produced in Russia.
Recent
History
• 2003 – Present : Diamonds synthesized by Chemical Vapour
Deposition (CVD) ; Ongoing advancements.
Tanzanite
Gemstone Refractometer

Origins of Gems:
Gemstones derive their origin from diverse geological processes shaping the Earth's crust
and mantle. Sedimentary, igneous, and metamorphic processes each contribute
uniquely:
i. Sedimentary rocks lack primary gem material except for organic substances like
Amber and jet but may form secondary deposits if weathered rocks carry heavier
minerals, seen in Brazil, Myanmar, and Sri Lanka as water-worn pebbles.
ii. Opals originate from silica-bearing water solidifying in rock crevices. Intrusive
igneous rocks house vital gem minerals like feldspar, quartz, and tourmaline,
forming sizable crystals through gradual cooling and fractional crystallization.
Pegmatites and geodes exemplify intrusive gem-bearing rocks.
iii. Metamorphic rocks yield gems like emerald, alexandrite, ruby, sapphire, garnet, etc.
through chemical reactions with molten magma. Shearing and crushing result in
gems like garnet and jadeite.
iv. Diamonds crystallize in the Mohorovičić discontinuity or Moho, a crust-mantle
transitional zone deep beneath the Earth's surface. They ascend through volcanic
eruptions, solidifying into kimberlite pipes. Over time, erosion transforms pipes into
hillocks, releasing diamonds into alluvial deposits along riverbeds and coastal strips.
In essence, gemstone genesis intertwines with sedimentary, igneous, and metamorphic
processes, showcasing the Earth's geological wonders that shape these treasures.
Minerals, Rocks and Gemstones:
Mineral: Formed within the Earth, inorganic
substance with constant properties. Only about
sixty minerals qualify as gem materials due to
beauty, rarity, and durability. Precious metals
like gold and platinum, along with diamond, are
chemical elements.
Rocks: Mixtures or aggregates of minerals,
forming the Earth's crust. Examples include
granite.
Gem Materials: Limited to about sixty minerals
meeting criteria for beauty, rarity, and
durability. Precious metals and diamond stand
out as chemical elements. Gemstones are
primarily single-mineral entities, with
exceptions like lapis lazuli
20XX 7

Introduction to
Gemmology Gems are found or mined in distinct geological settings, such as gem gravels,
gem-bearing veins, or diamond pipes. Location of discovery is crucial for
mineralogists as it offers insights into the processes of gemstone formation.
I. Direct Deposit: Many gemstones are discovered near their original
formation site. This type of deposit provides valuable evidence for
understanding the methods of gemstone formation.
II. Alluvial Gem Deposits: Result from gems being transported away from
their formation sites. Agents include weathering forces like wind or rain
and river flows. Alluvial gems often exhibit abrasions on surfaces,
indicating the distances traveled.
III. Residual Deposits (Eluvial): Gems released by weathering may settle
with little transportation or concentration. These deposits, known as
eluvial, showcase gems finding a resting place near their parent rocks.
Signs of Transport: Abraded surfaces observed in water-worn topaz pebbles
and rounded profiles of diamond crystals from Namibian coastline.
Geological Insights: The geological context of gemstones serves as a rich
source of information about their origins. Direct deposits and alluvial
displacement offer unique perspectives on the dynamic processes shaping
precious stones.

Hydraulic Mining: Water jets dislodge gem material from overburden.
Channels on hillsides direct pressurized water to split rocks, washing gem-
laden gravel downhill. Discontinued due to severe environmental impact.
River Panning: Collection of gemstones during river or stream gravel washing.
Pans filled with water settle heavy material, using riffles for gem separation.
Labor-intensive with lower success rates.
Open Pit Mining: Conducted under Gemologist guidance, removing layers to
expose rocks for gem extraction. Cost-effective, results in landfill areas after
mining completion
Strip Mining: Like open-pit mining, involving drilling, blasting, and sequential
strip removal. Strips filled with dirt, and top layer levelled for future land use.
Mountaintop Removal Mining: Involves clearing mountain tops, blasting with
dynamite, and retrieving gemstone-rich rocks. Displaced dirt is dumped into
valleys using bulldozers.
Quarrying: Environmentally less hazardous method used for building
materials. Rocks drilled or blasted for various uses, with gemstones extracted
during subsequent processing.
Borehole Mining: Deep holes drilled into the plain land, where water is
used to break down rocks and create a gemstone-bearing slurry.
Environmentally friendly and adaptable to shifting locations.
Drift Mining: Conducted on mountain sides, with horizontal tunnels or
drifts created. Gravity aids in the material's descent for easy retrieval.
Shaft Mining: Vertical tunnels or shafts created below mountains for
miners' movement and material extraction. Costly but effective method for
deep gemstone extraction.
Slope Mining: Shafts made slanting and parallel to the ground for situations
where straight shafts are challenging. Conveyor systems transport broken
rocks for processing.
Hard Rock Mining: Tunnels made inside the ground, often referred to as
Adits, with shafts extending vertically using explosives. Most dangerous
method, creating different floors for various purposes, including gemstone
extraction.

Crystallography
Determinants of Crystal Properties:
- Determined by symmetry and structure.
- Higher crystal symmetry implies more non-directional properties.
- Lower crystal symmetry results in more directional properties.
Crystal Symmetry and Space Lattice: Crystal symmetry = a regular
arrangement of atoms in three dimensions; linked to the space lattice.
Described by a unit cell, the smallest repeating unit of the lattice. 14
Bravais lattices categorize any crystal structure. These lattices belong to
seven crystal systems based on unit cell shape and angles.
Understanding crystallography principles is crucial for gemstone
professionals, guiding them in cutting, cleaving, and polishing processes.
The arrangement of atoms in crystalline minerals and the forces
governing this arrangement play a pivotal role in determining the
physical and optical properties of gemstones.
Crystallography is the scientific study of the
arrangement of atoms in solids, especially
crystals. Crystals are solids that have a regular and
repeating pattern of atoms or molecules, forming
a three-dimensional structure called a crystal
lattice. Crystallography can reveal the shape, size,
symmetry, and composition of crystals, as well as
their physical and chemical properties.
Crystallography is an important branch of science
that has applications in many fields, such as
materials science, solid-state physics, chemistry,
biology, geology, and medicine.
Use in Gem Cutting and Polishing:
- Diamond cleavers leverage crystallography
knowledge to split stones along cleavage
planes.
- Lapidaries consider crystallography when
placing facets to avoid directions of maximum
hardness and enhance color in gemstones.
- Avoiding polishing facets parallel to
cleavage planes prevents uneven surfaces or
gem cleavage during polishing.
2023 10
Importance of Crystallography

The atomic structure
of a crystal
Illustrating the concept of minute elementary blocks, Haüy proposed crystals grow through the
assembly of tiny units or building blocks. This theory finds practical validation in the cleavage
rhombohedron of Iceland spar calcite, which, when broken down, reveals identical copies of the
parent structure down to the molecular level.
The fundamental building block, termed the "unit cell," represents the smallest lattice structure
retaining all mineral characteristics, shaped by chemical composition and valency forces.
Crystal nucleation and growth involve processes akin to salt crystal formation from a saturated
solution, emphasizing factors such as saturation, evaporation, and cooling. The parallel extends to
mineral crystal growth from molten or aqueous solutions in the Earth, influencing crystal sizes in
rocks and the formation of geologically recognized cavities like geodes, druses, or vugs.
In 1669, Nicolas Steno discovered that
quartz crystal interfacial angles remain
consistent across crystals, a groundbreaking
revelation extending to various minerals.
Abbé Haüy deemed the 'father of
crystallography,' mathematically established
that each substance possesses a distinct,
unique form, postulating orderly internal
structures behind their external conformity.
René Just “Abbé” Haüy
1743-1822

All solid matter falls into the categories of either crystalline or non-
crystalline, or a combination of both. The term "crystalline" originally
derived from the Greek word "krystallos," meaning ice, later expanded to
describe any substance with the clarity of ice, even if not colorless or
transparent.
Non-Crystalline Substances: Atoms and molecules are randomly
arranged, lacking a specific order or pattern. Cannot develop a
characteristic shape due to the absence of alignment. Examples include
glass, amber, and jet, with glass containing disordered silicon tetrahedra
frameworks.
Crystalline Substances: Atoms and molecules exhibit an ordered and
symmetrical three-dimensional pattern. External shape often reflects the
underlying crystal structure, visible in mineral specimens. Examples
include most minerals, with notable exceptions like opal, and various non-
mineral substances that can form crystals.
Non-Directional Properties (Scalar/Isotropic):
Independent of measurement direction.
Examples: density, specific heat, thermal
conductivity, and electrical resistivity. Associated
with metals featuring non-directional or isotropic
bonding.
Directional Properties (Tensor/Anisotropic):
Dependent on the measurement direction.
Examples: refractive index, elastic modulus,
piezoelectricity, and optical activity. Associated
with non-metals like insulators and
semiconductors with directional or anisotropic
bonding
Directional Properties in Crystalline Materials:
- Physical characteristics vary with crystal
orientation, a feature absent in non-crystalline
substances.
- Diamond serves as an example with
directional-dependent properties like cleavage
and hardness.
- Optical properties, including color, can also
vary with viewing direction in colored gems like
ruby and sapphire.
Crystalline and Non-Crystalline Materials

Symmetry in Crystals
In crystallography, symmetry is crucial for defining the structure of crystals,
especially in gemmology. Gem crystals are classified into seven systems
based on their varying symmetry levels, ranging from highly symmetrical
cubic to less symmetrical triclinic systems. While gemmological tasks focus
on recognizing crystal habits and understanding optical and physical traits,
a solid grasp of imaginary crystallographic axes and symmetry elements is
essential. Imaginary crystallographic axes act as a reference for describing
a crystal's idealized shape, intersecting at the 'origin' point and aiding in
identifying crystal faces based on angles and lengths. Three essential
elements of symmetry contribute significantly to crystallography. These
elements include:
I. Axis of Symmetry: An axis around which a crystal can be rotated to a
position where it appears unchanged.
II. Plane of Symmetry: A plane that divides a crystal into two identical
halves, reflecting symmetrical properties.
III. Centre of Symmetry: A point within the ideal crystal where lines
drawn from any point through the center to the opposite point result in
identical lengths and angles.
PRESENTATION TITLE 20XX 13
An axis of Symmetry
A plane of Symmetry

Crystal systems
The Seven Crystal Systems and their Elements of Symmetry As mentioned earlier, crystals can be grouped into seven
systems. These can be further subdivided into 32 classes. The subdivision is based on the different degrees of
symmetry (as specified by the elements of symmetry) of the crystals within each system. The seven crystal systems
themselves are classified in terms of the number of their crystal axes, their relative lengths, and the angles between
them. The elements of symmetry included in the following descriptions of the seven crystal systems refer to the
highest level of symmetry in each case (it should be remembered that crystals do form with lower levels of symmetry
and may sometimes have no center of symmetry at all).

The Cubic System
Crystals in this system have the highest order of symmetry and
are also called isometric.
The cubic system has three crystal axes (a1, a2, a3), all of which
are of equal length
(a1 = a2 = a3) and intersect each other at right angles (90°).
Axes of symmetry: 13 (six 2-fold, four 3-fold, three 4-fold)
Planes of symmetry: 9
Centre of symmetry: 1
Common forms: cube, eight-sided octahedron, twelve-sided
dodecahedron
Examples: diamond, garnet, spinel, fluorite

The Tetragonal System
This has three crystal axes. The two lateral ones are of equal
length (a1 = a2) and at right angles (90°) to each other. The third
(principal or c) axis is at right angles (90°) to the plane of the
other two and is shorter or longer than them (c ≠ a1)
Axes of symmetry: 5 (four two-fold, one four-fold)
Common forms: four-sided prism with square cross-section
Examples: zircon, scapolite

The Hexagonal System
Contains four crystal axes, the three lateral ones are of equal
length (a1 = a2 = a3) and intersect each other at 60° in the same
plane. The fourth (or principal) c-axis is at right angles to the
other three and is usually longer.
Axis of symmetry: 7 (six two-fold, one six-fold)
Common forms: six-sided prism
Examples: beryl, apatite

The Trigonal/Rhombohedral System
This system (sometimes treated as a subdivision of the hexagonal
system) has four crystal axes which are arranged in the same
manner as in the hexagonal system. The symmetry of the trigonal
system is, however, lower than that of the hexagonal system.
Axes of symmetry: 4 (three two-fold, one three-fold)
Common forms: three-sided prism, rhombohedron
Examples: calcite, corundum, quartz, tourmaline

The Orthorhombic System
This system has three crystal axes, all at right angles (90°) to each
other and all having different lengths (a b c). The principal or c
axis is the longest, and of the remaining two lateral axes, the
longer b is known as the macro axis and the shorter a is called the
brachy axis.
Axes of symmetry: (three two-fold )
Common forms: rectangular prism (prism with cross-section of
playing card ‘diamond’), bipyramid comprising two four-sided
pyramids joined at the base
Examples: topaz, peridot, chrysoberyl, andalusite, sinhalite,
zoisite

The Monoclinic System
There are three crystal axes in this system, all of unequal lengths
(a ≠ b ≠ c). The b axis, known as the ortho axis, is at right angles to
the plane of the other two which cut each other obliquely. The
longest of these is the c axis, and the one inclined to it (at an
angle other than 90°) is called the a or clino axis.
Axes of symmetry: (one two-fold )
Common forms: prisms and pinacoids
Examples: orthoclase feldspar (moonstone), diopside

The Triclinic System
This system has three crystal axes, all of unequal lengths (a b c)
and all inclined to each other at angles other than 90°. The longer
lateral axis is called the macro, and the shorter is called the
brachy as in the orthorhombic system
Axes of symmetry: none
Planes of symmetry: none
Common forms: prism (tilted sideways and backwards) with
pinacoids
Examples: plagioclase feldspar, microcline feldspar (amazonite),
rhodonite, turquoise (usually in microcrystalline aggregates)

Crystal Form:
A crystal form is composed of a group of crystal faces which are
similarly related to the crystal axes.
A form made up entirely of identical interchangeable faces is
called a closed form (e.g. a cube or an octahedron).
A form which is only completed by the addition of other forms is
called an open form. An open form cannot exist on its own and
must be completed by the addition of other open forms which act
as suitable terminations to complete shape.

Crystal Habit:
The shape in which a mineral usually crystallizes is referred to as its habit.
From the sketches illustrating the seven crystal systems it can be seen
that minerals belonging to the same system can often have very different
habits, despite the similarity of their internal crystal structures.
These differences in habit can be the result of combinations of more than
one form. Other differences may be due to the variety of terminations
which some crystals adopt.
Some of the more bizarre shapes are caused by the parallel growth or
interpenetration of two or more crystals.
The many differences in crystal habit can be confusing at first encounter,
as it is often difficult to associate a habit with the idealized shape of one
of the seven systems. This very factor can sometimes be an advantage,
however, as it can be used to identify a mineral simply by the individuality
of its habit. Sometimes the shape of a crystal is due to influences which
modify its normal habit
Examples of a few crystalline habits:

Twinned Crystals:
A twinned crystal consists of two or more individual crystals that
have grown together in a crystallographic relationship to produce a
symmetrical shape.
Twinning, which is very common with quartz crystals, usually occurs
in one of two forms:
A. Contact Twins: These occur when the twin-halves of a crystal
have grown with one half rotated through 180° to the other
half. Spinel and diamond often occur in this form. With
diamond, a contact twin is called a macle.
B. Interpenetrant Twins: consist of two or more crystals which
have grown in proximity and have penetrated each other with a
direct relationship between their axes. Common examples can
be seen in the interpenetrant cubes of fluorite, and in staurolite
twins.
Contact Twins
Interpenetrant Twins

Gemstone
Examination
• Physical Methods
• Fluorescence and Spectroscopic
techniques
• Optical Methods
• X-Ray Diffraction Studies
GEMMOLOGY 2023 25

Cleavage, in mineralogy, refers to the propensity of a crystallized mineral to break along
specific directions, yielding relatively smooth surfaces. This phenomenon results from
weak cohesive forces within the mineral along different directions and is influenced by
the mineral's form and crystal structure.
Crystals may exhibit cleavage in more than one direction, and a crystal cleaving in one
direction will do so equally well in any parallel plane. Cleavage planes align with the
crystal structure and are parallel to potential crystal faces. This splitting is attributed to
the regular arrangement of atoms, rendering amorphous substances devoid of
cleavage.
Physical
Properties:
Cleavage in
Gemstones
Identification of cleavage:
• observing minutely stepped flat planes or a wavy pattern
on the mineral's surface.
• The presence of a pearly luster indicates its laminated
nature.
• Iridescent colors along cleavage directions or cracks may
develop due to air penetrating flaws.
• Smooth parallel cracks or directional cracks in cut
specimens signify cleavage directions. A gem cutter
considers cleavage when selecting a cut type to avoid
flaws or cracking.
• Cleavage is especially useful in cleaving rough stones like
diamonds to enhance yield and reduce costs.
•Octahedral cleavage occurs parallel
to octahedral faces in cubic crystals
(e.g., diamond).
•Prismatic cleavage aligns with the
prism face (e.g., spodumene).
•Basal cleavage is parallel to the
basal pinacoid (e.g., topaz).
•Rhombohedral cleavage occurs
parallel to rhombohedral faces
(e.g., calcite).
•Perfect cleavage is exemplified by
minerals like mica, topaz, or calcite.
•Easy but imperfect cleavage, as seen in
octahedral cleavage in fluorite.
•Easy and perfect cleavage, exemplified
by rhombohedral cleavage in calcite.
•Difficult cleavage, as observed in
peridot.
•Perfect but difficult cleavage, such as
octahedral cleavage in diamond.
•Difficult and imperfect cleavage, like the
basal cleavage in beryl.
Types of Cleavage: Types of Cleavage based on Quality:

Cleavage is a directional feature and can only exist in crystalline substances. Cleavage occurs in a gemstone
as a well-defined plane of weak atomic bonding which allows the stone to be split in two leaving reasonably
flat surfaces. These cleavage planes are always parallel to a crystal face in a perfectly formed single crystal.
The gemstones that possess the property of cleavage, the cleavage planes are the result of the atoms lying
parallel to these planes being more closely linked together than the atoms between the planes. The bonding
force along the planes of atoms is therefore much stronger than between the planes.

Physical
Properties:
A. Parting
B. Fractures
Parting (false or pseudo-cleavage):
Some minerals which do not possess cleavage properties can be divided in two along a
plane of weakness rather than along a plane related to the stone’s crystal faces. Such a
direction of weakness is called a parting plane. Parting is usually caused by a form of
twinning. In the case of corundum and labradorite, parting occurs along directions of
lamellar twinning.
A significant difference between parting and cleavage is that Parting only occurs at
discrete intervals along the twinned crystal, while cleavage planes are separated only by
atomic layer intervals.
• Parting is usually due to lamellar twinning, as in labradorite feldspar.
• Parting is possible only along definite planes in twinned crystals and not along every
parallel plane.
• A crystal may exhibit both cleavage and parting, as in corundum, (parting parallel to the
rhombohedral face and cleavage parallel to the basal pinacoid) and labradorite (parting
along the lamellar plane and two other directions of cleavage).

Fracture:
The way in which a mineral breaks (other than by cleavage or parting) is known as fracture. Unlike cleavage and parting,
fracture occurs in a random direction, often as the result of a sharp impact. It is typical of non-crystalline materials such
as glass but can also occur in those crystalline materials which do not have significant planes of cleavage or weakness
(e.g. quartz and beryl). Fracture can also happen occasionally with other crystals, even (as indicated earlier) with
diamond.
Sometimes the surface contour of a fracture can be distinctive enough to constitute a useful identifying feature
Types of fracture, together with the gem materials in which they occur,
are as follows:
• Conchoidal: This is a shell-like fracture consisting of a series of
scalloped rings similar to those seen in a seashell. Conchoidal
fractures occur in quartz and the garnets. It is particularly
characteristic of glass.
• Hackly or splintery: This takes the form of long fibrous splinters and
can be seen in nephrite, jadeite and ivory.
• Smooth or even: This type of fracture, although not perfectly flat,
shows no signs of significant irregularities. Examples of smooth
fractures can often be seen in rough diamonds

Physical
Properties: In gemmology and mineralogy, it is a measure of a gemstone's ability to
resist abrasion. A gemstone's hardness must be taken into consideration
for durability, in addition to rarity and beauty, when selecting it for use in
jewellery. A gemstone's ability to resist abrasion is not due to the hardness
characteristic of its atoms, but rather to the strength of the bonds
between these atoms. Microscopic particles of silica or quartz in airborne
dust constantly test a gem's durability by gradually abrading its polished
surface.
The reason for this is that the gem must be capable of withstanding the
wear and tear to which jewellery is subjected in everyday use.
While a ring-mounted gemstone may have to endure the occasional hard
knock, it is the presence in airborne dust of microscopic particles of silica
or quartz that is the real test of a gem’s durability. These particles are in
continual contact with the gem, and if the stone is relatively soft they will
gradually abrade its polished surface.
• Hardness
• Specific Density
• Density and Relative
Density

Mohs' Scale of Comparative Hardness:
Mohs' scale is a method for assessing the hardness of minerals, developed by German mineralogist Friedrich Mohs in
1822. It measures a material's resistance to abrasion when subjected to a pointed fragment of another substance at a
pressure lower than that causing cleavage or fracture. The scale is purely comparative, with divisions not evenly spaced.
Friedrich Mohs chose ten common minerals as standards, numbered from one to ten based on ascending hardness.
These minerals were selected for their distinctive hardness and easy availability in a high state of purity. The chosen
minerals are:
While hardness is crucial for durability, it is not the sole factor.
Brittleness also plays a role; for instance, Zircon, despite its hardness
of 7 to 7.5, can be brittle, leading to 'paper wear.’
In contrast, jade minerals (nephrite and jadeite) exhibit toughness due
to their microscopic interlocking fibers or crystals, making them
durable tools for prehistoric man.

• Hardness pencils, with pointed fragments of minerals adhered to
wood/metal holders, follow Mohs' scale standards. Starting with
the softest pencil and progressing, the gem is scratched, and the
hardness lies between the scratches.
• Reverse hardness test: A safer alternative, where the gem's facet
edge or girdle is drawn across plates of known hardness. The
stone's hardness falls between the unscratched plate and the
preceding one.
• Hardness plates: made from orthoclase, quartz, synthetic spinel,
and synthetic corundum are used for testing. Synthetic corundum
sections serve as a confirmatory test for diamond.
• Scratch Test on Glass Slides: A series of scratch tests on a glass
microscope slide using minerals provides insights into relative
hardness via varied depths of scratches . A glass microscope slide
serves as a simple, non-destructive reference guide for estimating a
gem's hardness.
Hardness Pencils

Physical
Properties:
Specific Gravity (SG): It represents the ratio of the weight of a substance to the
weight of an equal volume of pure water, measured at standard atmospheric
pressure and 4°C, the temperature at which water is most dense. SG is a
dimensionless ratio, expressed without units. The SG of a material like diamond,
for instance, being 3.52, means it is 3.52 times heavier than an identical volume
of water. Water is the chosen standard with an SG of 1.0.
Density: It is the weight per unit volume of a substance, measured in units of
weight and volume. In the International System of Units (SI), density is
expressed in kilograms per cubic meter (kg/m³). For diamond, the density is
3520 kg/m³, equivalent to 3.52 g/cc in older textbooks.
Relative Density: In the context of liquids, relative density, like SG, is the ratio
between the weight of the liquid and the weight of an equal volume of water
at 4°C.
• Specific Gravity
• Density & Relative
Density

Measurement of Specific Gravity:
Principle: based on Archimedes’ principle which states that a body submerged in a fluid experiences an upward buoyant
force equal to the weight of the fluid it displaces.
Calculation of Specific Gravity (SG):
However, for use with gemstones due to their
comparatively smaller volumes, more accurate SG
bottle, called a pycnometer, consists of a small glass
phial having a ground-glass stopper through which runs
a capillary channel.
At the gemmologists' laboratories, more sophisticated methods are used,
like single-pan, two-pan, and spring balance method for SG estimation.
A pycnometer
Single-pan SG balance Two-pan SG balance Spring Balance method

Optical
Properties:
• Colour
• Lustre
• Sheen
• Reflection & Refraction
• Polarization & Pleochroism
• Luminescence

Colour of Gemstones:
The beauty of appearance was given as a prime quality of a gemstone. In a world where colour is
one of the dominant visual sensations, it is not surprising that a gemstone’s beauty is largely
determined by its colour. Although the perception of colour is an everyday experience, and as
such is taken for granted, in gemmological studies it is important to understand exactly how the
appearance of colour is produced in a gemstone. The following are some ways which produce
“colors” in gemstones:
I. Selective absorption and Gemstone coloration.
II. Color-change gemstones and Alexandrite effect.
III. Coloration due interference & dispersion.
IV. Color-centres and gemstone coloration.

Selective
Absorption &
Gemstone
Coloration:
Selective absorption: The suppression
of certain wavelengths or colors from
the white light illuminating an object is
known as selective absorption. It can
be visually analyzed using an
instrument called a spectroscope.
37
A gemstone having an absorption
band in the blue/violet end of the
spectrum (such as a cape series
diamond) the residual red and
green part of the white light
reflected from the stone gives it a
yellow appearance
White light consists of a balanced mix of all visible spectrum colors. When observing a
colored gemstone in white light, the perceived color results from the stone selectively
absorbing specific wavelengths or bands of wavelengths from the original light. In
transparent stones, absorption occurs as light passes through, while in opaque stones, it
happens during reflection from the surface layer. The overall color appearance stems from
the combined effect of the remaining unabsorbed parts of the white light spectrum,
creating a complementary color to the absorbed wavelengths.
An absorption spectrum showing
three bands (caused by iron) which
are diagnostic for blue sapphire
The Visible Spectrum

Allochromatic Gemstones:
• These exhibit selective absorption due to
impurities present in their composition.
• Ex- Cr oxide in ruby or Fe oxide in aquamarine
are examples of impurities causing coloration.
• Most colored gemstones fall under this
category.
• In their pure state, some allochromatic gems
exist as colorless varieties. Rock crystal,
colorless sapphire, topaz, etc. have no intrinsic
coloration.
Idiochromatic Gemstones:
• These owe their color to chemicals within their
own composition.
• Ex- Cu in turquoise or Mn in spessartite garnet.
• Idiochromatic gems derive color from their
inherent chemical makeup, distinguishing them
from allochromatic counterparts influenced by
impurities.
Selective absorption is mainly caused by 8
metallic transition elements. In allochromatic
gems, these elements act as colouring impurities,
while in idiochromatic stones, they are integral to
the mineral's chemical formula. Zircon and
certain topaz, quartz, and fluorspar varieties lack
measurable transition elements, making their
colors more susceptible to change through heat
and irradiation.
Tints Vary Chromatically, Man I Could Never Copy!

Colour-Change
Gemstones &
Alexandrite
Effect:
Alexandrite Simulants: Due to its rarity, various simulants
have been introduced to mimic alexandrite's color-change
effect, such as synthetic corundum doped with vanadium and
green synthetic spinel.
Synthetic Alexandrites: True synthetic replicas of high-quality
Siberian alexandrite were introduced in 1973, but they are
costlier than corundum-based simulants. Identifying and
distinguishing synthetic alexandrites from natural gemstones
is a challenge for gemmologists.
Occurrence in Other Stones: Although rare, colour-change
can be observed in other natural stones like corundum,
spinel, and garnet, requiring careful examination for accurate
identification.
Color-change gemstones exhibit
alterations in body color when
subjected to different lighting
conditions. This phenomenon,
known as the alexandrite effect,
is most notably observed in the
rare alexandrite variety of
chrysoberyl.
Alexandrite, a prime example,
appears red in incandescent
light and green in daylight
20XX 39

Dispersion is an optical property in many gemstones that contributes to color.
White light refracts at varying degrees upon entering and leaving a gemstone,
creating prismatic colors. Higher degrees of dispersion produce flashes of
colored light, known as "fire."
The degree of dispersion correlates with refractive index, measured by the
difference at the B and G Fraunhofer wavelengths. Diamond has a high refractive
index but modest dispersion. Prismatic colors are most noticeable in colorless
gems but can still be seen in colored stones like demantoid, garnet, and sphene.
Gemstone Coloration due to Interference:
Gemstone coloration can arise not only from chemical composition but also
from optical effects. One such effect involves the interference between rays
reflected from a gemstone's surface layers. This interference generates
complementary colors, akin to selective absorption. Alternatively, if the rays are
in phase at another wavelength, the reflected color is intensified. This light
interference effect significantly contributes to the coloration of gemstones like
opal, labradorite, and moonstone.
Gemstone Coloration due to Dispersion:

Gemstone Coloration due to Color-centers:
Gemstones get their unique colors from defects in their crystal lattices called color centers. These can be induced by
natural or artificial irradiation, resulting in ‘electron’ or 'hole' color centers.
Examples include fluorite, quartz, and certain diamonds.
Fancy colored diamonds may result from natural irradiation, while zircon colors may stem from impurity atoms.
Yellow Cape series diamonds get their color from nitrogen replacing carbon, while rare diamonds like the Dresden
Green have a uniform green color due to natural radiation damage.
Pink, mauve, and brown diamonds' colors are attributed to plastic deformation, while blue diamonds acquire their
color from boron atoms replacing carbon.

Lustre of a gemstone is the optical effect created by the reflectivity of the stone’s surface. Lustre
is directly related to the refractive index of gem material, and although the lustre of some
gemstones is visible in the rough, its full potential is usually revealed only when the stone is
polished. Because gemstones cover a wide range of refractive indices from 1.43 to 3.32, they
also exhibit different degrees of lustre.
In recent years, an instrument called a reflectance meter has made it possible to provide a comparative
measurement of a stone’s lustre and to use this as a means of identification

Lustre refers to surface reflectivity while sheen results from
light rays reflecting beneath a gemstone's surface. There are
different types of sheen:-
• Chatoyancy: creates a band of light caused by reflection from
parallel groups of fibers or channels within the stone. Tiger’s-
eye displays this effect, with the brightest chatoyant 'line'
found in cymophane, a variety of chrysoberyl.
• Asterism: produces a 'star' effect seen in rubies and
sapphires due to fine parallel fibers or crystals intersecting at
60°.
• Iridescence: presents a play of rainbow-colored light caused
by thin layers beneath the gemstone's surface.
Labradorescence, seen in labradorite and spectrolite, and
adularescence, observed in moonstone, are specific forms of
iridescence.
Asterism in a Ruby
Chatoyancy sheen effect

Reflection &
Refraction
Effect of Color on Gemstone
Transparency:
Color plays a significant role in
determining the transparency of a
gemstone. Darker colored stones
allow less light to pass through, while
thicker stones result in greater light
loss. Transparent allochromatic gem
minerals are difficult to identify based
on color alone. Opaque gemstones
with distinctive color and surface
patterning are more easily
recognizable than transparent ones.
20XX 44
Reflection of light is a phenomenon where light
waves encounter a surface and bounce back. When
light strikes an object or a boundary between two
media, a portion of the light is thrown back into the
same medium it came from.
Refraction of light is the bending or change in
direction that occurs when light passes from one
medium into another with a different optical density.
The speed of light changes as it moves through
different substances, causing the light rays to alter
their path.
The refractive index (RI) is the most important factor when identifying
gemstones. The RI of most gemstones is a constant that can be measured
accurately to two decimal places, making it easy to distinguish between gems
with very little difference in their RI.
Refractive index is a numerical index that describes the relationship between
the angle of incidence in air and the angle of refraction in the material.
𝑛 =
sin i
sin 𝑟
=
𝑣2
𝑣1

20XX 45
Critical Angle:
Minimum angle for total
internal reflection to
occur, when light passes
from higher to lower
refractive index
medium.
𝑛 =
1
sin 𝑐
Total Internal
Reflection: Light within a
medium strikes another
medium boundary at angle
greater than critical angle,
causing total reflection into
original medium instead of
refraction into second
medium.
The instrument used to measure refractive index (RI). A refractometer
measures the extent to which light is bent when it moves from air
into a sample.
The refractometer has five main components: a rear window, hinged
cover, hemicylinder, viewing lens, and scale.
• The rear window lets light into the device and may have a
polarizer. The hinged cover allows you to place the gemstone on
the flat side of the hemicylinder.
• The hemicylinder is a glass tube that has two chambers, one for
the gemstone and another for a liquid with a known refractive
index.
• The viewing lens lets you see the shadow of the gemstone on the
scale, which shows refractive index values for different colors or
wavelengths of light. You read the scale by looking at where the
shadow line falls on it.
A schematic of a refractometer

1. The refractometer works by the use of a glass hemicylinder
inside that has a flat surface extending into the body of the
refractometer .
2. When light is introduced into the back of the unit, it travel
through the glass hemicylinder and up to the gemstone,
where the stone is in optical contact with the glass by use of
a refractive index liquid.
3. This liquid keeps air from between the stone and the glass
hemicylinder and allows light to travel at the same speed
through the glass, gets refracted by the gemstone, and then
back through the glass hemicylinder to the viewing lens.
4. The amount of slowing down or bending of the light that the
gemstone does to the light beam, causes a light and dark
area joined by a thin green line as shown below.
5. This green line is the refractive index reading of the
gemstone.
Working of a Refractometer

The lustre, reflectivity or reflectance of a
gemstone can be described qualitatively as
adamantine, vitreous, resinous, etc.
However, it can also be measured in
absolute terms as the ratio between the
intensity of the reflected ray and that of the
incident ray:
The degree of luster or reflectivity of a gem
(assuming a ‘perfect’ polish) is due mainly
to its refractive index but is modified by
other factors such as its molecular structure
and transparency.
20XX 47
Fresnel’s Reflectivity Equation:
Principle: The principle of reflectivity came into use due to the need for fast
identification methods for gemstones with RI over the limits of a standard
refractometer.
Parts of a Reflectance Meter:
• Light source: Solid-state IR LEDs
• Detector: Sealed silicon photodiode.
• Sensory Slot: Here, Gemstone is placed.
• Meter: Displays the Reflectivity of the gemstone.
• Light Shield: Placed over the gemstone to cut-off stray light.
• Toggle Switch: Used to move between lower & Upper Scale of R.Is.
• Battery: Usually, a 9V battery.
Hanneman ‘Jeweler’s Eye’ reflectance meter
Sketch of Jeweller's Eye

Working of a Reflectance Meter:
• Center a clean, polished facet of a gem over the
1/32" light sensor hole.
• Use the provided light shield to cover the gem.
• Press the red button to activate the Jeweler’s Eye.
• The needle points to the gemstone based on
reflectivity.
• Ensure the gem is of sufficient size and has a table
facet of at least 1/32".
• Additional scales are available for less common
gems on the case.
• The device is portable, operates on a single 9-volt
battery (included), and comes with a carrying case,
accessory kit, and recalibration tool.
Circuit Diagram of “Jeweller's Eye”

Double Refraction
in Crystals:
Some materials like glass, amber, and cubic crystal system
gemstones have one refractive index, leading to a single
refracted ray. However, crystalline materials (gemstones)
from various crystal systems possess two refractive indices,
split into two rays, polarized at right angles, known as double
refraction or birefringence.
The two polarized rays in birefringent materials travel at
different speeds through the crystal and undergo different
amounts of refraction. Gemstones with high double
refraction, like zircon, exhibit a noticeable doubling of the
image of back facet edges when viewed through the main
facet using a hand lens.
Understanding the degree of double refraction and the
specific values of each refractive index aids gemologists in
identifying gemstones. This optical property provides a
unique characteristic that contributes to the overall
assessment and recognition of gemstone types.
Unpolarized incident light (vibrating in all
directions) is split into two plane polarized
rays on entering a doubly refracting
materia
20XX 49

o Refers to the process of transforming unpolarized light waves, which
vibrate in all directions perpendicular to their travel path, into
polarized light which vibrates in single planes. In a doubly refracting
material, such as certain gemstones, unpolarized light passing through
emerges as two separate polarized rays.
20XX 50
POLARIZATION
PLEOCHROISM
o An optical phenomenon observed in certain colored gemstones that are doubly refracting. When light enters these
stones, it splits into two separate rays polarized at right angles. In pleochroic gemstones, these rays may emerge with
differing shades or colors due to differential selective absorption by each ray.
o If two colors/shades are produced, the stone is termed dichroic, while trichroic stones exhibit three colors/shades.
Pleochroism serves as a valuable tool in identifying anisotropic minerals and can enhance the aesthetic appeal of
certain gemstones
Polarization & Pleochroism

The Polariscope:
A polariscope is an optical tool that uses polarized
light to analyze stress distribution in transparent
materials. It helps evaluate structural integrity
during materials testing and quality control.
The polarized light microscope consists of a light
source, a polarizer, a sample, an analyzer, a rotating
stage, and an observation point/viewer.
The light source provides a stable beam of light,
the polarizer produces polarized light, the sample
exhibits stress patterns, the analyzer controls light
intensity, the rotating stage rotates the sample, and
the observation point/viewer allows the observer to
view the sample.

Working of a Polariscope:
Principle: The interaction of polarized light with the
stressed sample creates interference patterns, revealing
information about the internal stresses within the material.
Adjusting the orientation of the polarizer and analyzer
helps in visualizing different aspects of the stress patterns.
To observe a gemstone, place the polarizer and analyzer in
crossed position, and turn on the light source. Slowly turn
the stone while observing it through the analyzer. There
are four possible outcomes:
1. Stone appears dark throughout rotation.
2. Stone is isotropic with single refractive index.
3. Stone blinks 4 times, appearing light and dark, which
means it's anisotropic with double refractive index.
4. Stone is a microcrystalline or cryptocrystalline
aggregate, showing anomalous double refraction (ADR)
and is isotropic (single refractive).a a

A dichroscope is an optical instrument used in gemmology
to analyze a gemstone's colors when viewed from different
crystallographic axes, helping gemmologists identify
pleochroism.
Principle: The dichroscope relies on the differential
absorption of light by a gemstone’s crystal structure.
Different crystallographic axes absorb light of different
wavelengths, resulting in color variations when viewed
through the dichroscope.
Construction: It typically consists of two calcite prisms or
polarizing filters that are joined together. These prisms
separate the incoming light into two rays, allowing the
gemstone’s pleochroic colors to be observed separately.
Two Calcite Dichroscopes
Construction Sketch of a dichroscope

Luminescence
Luminescence refers to the emission of light radiation by certain materials when they absorb energy but
do not reach the point of incandescence or burning. The luminescent effect involves the excitation of
atoms within the material. The surplus energy causes electrons to move from their stable orbits (ground
state) to higher energy levels (excited state). When these electrons return to their stable orbits, they
release the surplus energy in the form of electromagnetic radiation, often in the visible spectrum.

Gemstone
Luminescence
Luminescence is the phenomenon that shows itself as a "glowing" of a
gemstone. This is caused by absorption of energy and the releasing of surplus
of this energy in small amounts.
The sources of energy are usually ultraviolet light, X-ray light, and even visible
light. When energy comes from light, it is referred to as photoluminescence.
In gemology we are usually only concerned with the following types of
luminescence:
1. Fluorescence
2. Phosphorescence
3. Triboluminescence
4. Tenebrescence (not technically a luminescence)
5. Cathodoluminescence
6. Thermoluminescence
The causes of luminescence are varied but are mostly due to impurities
("activators") or due to defects in the crystal lattice. In general, the presence
of iron inside the gemstone kills or suppresses luminescence.
Photoluminescence & Stokes’ Law:
Photoluminescence involves the
emission of visible light in response
to excitation by electromagnetic
radiation, such as visible light, UV
radiation, or X-rays.
Stokes’ Law specifies that the
resulting luminescence always has a
longer wavelength than that of the
original excitation energy.
Photoluminescence is a key feature
for gemmologists, aiding in the
identification of gemstones.
20XX 55

Phosphorescence: is a type of luminescence where there is a discernible delay between the absorption of energy and the
emission of visible light. The afterglow effect is observable when the excitation source is switched off. It occurs due to
the random movement of electrons between their normal and higher energy level orbits. The delayed release of surplus
energy distinguishes phosphorescence from fluorescence.
Fluorescence: is a type of luminescence where the emission of visible light occurs immediately upon exposure to an
excitation source. The luminescent effect ceases as soon as the excitation source is turned off. The movement of electrons
between energy levels happens promptly and in a random fashion. Fluorescence is characterized by its immediate
response to excitation.
Triboluminescence: is the emission of light from Some gemstones, like diamonds, when subjected to pressure or
abrasion. It's different from other types of luminescence because it's not caused by light, but by electric charges, which is
why it's also called electroluminescence. This effect is often seen when diamonds are cut, producing a red or blue glow.
Cathodoluminescence: Materials may display fluorescence when bombarded with electrons in a vacuum, often used for
research and analysis.
Thermoluminescence: The ability of certain gemstones to retain the energy they acquire from electromagnetic
radiation, and then release it again when heated. This effect is also called thermophosphorescence.

20XX 57
Tenebrescence is a unique property in some minerals and phosphors that causes them to darken in
response to radiation and reversibly bleach on exposure to a different wavelength.
It is not technically a luminescence, but an unstable color caused by low energy artificial irradiation from
the UV light source.
A tenebrescent stone requires an external energy source (light) to exhibit its colour, while luminescence is
a release of "stored" energy.
Examples include hackmanite, sodalite, tugtupite, spodumene, and chameleon diamonds.
Chameleon diamonds are olive-coloured diamonds that temporarily change colour when stored in
darkness or gently heated, similar to reversible photochromism.
Hackmanite is transparent when
extracted but adopts a purple shade after
sun exposure, a change that is also
reversible.

20XX 58
Gemstone Fluorescence:
Ultraviolet light (UV) is the most used excitation source. We cannot see UV light as it sits just below the visible light spectrum (400nm-
700nm) at 10-400nm. UV light enables us to see fluorescence because a gem material will absorb this radiation source and then emit
light that is lower in energy and therefore visible to the eye.It has to do with electrons. When electrons are excited by a source of
radiation, they jump to a higher energy level around the nucleus of the atom. The excited electron remains in this excited state for a
short period of time until it falls back to its original ground state. As the electron returns to its ground state, it emits energy either as heat
or as visible light (fluorescence).
Using UV Light to Test Gemstones
UV light testing uses two types of light: long wave (LWUV) with a wavelength of 365nm and short wave (SWUV) with a wavelength of
254nm. Equipment ranges from UV keyrings to UV cabinets. When testing gemstones, use caution with SWUV as it can be more
dangerous than LWUV and always wear protective UV goggles or ensure your UV cabinet has a filtered eyepiece to protect your eyes
from damage.
Synthetic Verneuil Sapphire, Scapolite,
Natural Sapphire, Topaz & Citrine
Under S-UV Under L-UV

Fluorescent reactions of some gemstones:

Spectroscopic
Techniques in
Gemmology
In gemstones, color often originates from the presence of 8 transition elements,
either as trace impurities or integral components of the gem's chemical
composition. In either case, these elements selectively absorb certain wavelengths
of light, contributing to the gem's distinct color. A spectroscope is used to analyze
the absorption spectrum of a gem, spreading its light into spectral colors and
revealing dark lines or bands where specific wavelengths are absorbed.
Absorption and Emission Spectra:
A spectroscope disperses light into its spectral colors. This process reveals dark
lines or bands that make up the absorption spectrum and help to identify
absorbed wavelengths in the gemstone. Even unpolished specimens can be
positively identified by the characteristic absorption features of transition
elements. It is worth noting that different gemstone species with the same
transition element can exhibit distinct absorption positions, which enhances the
accuracy of identification.
In some cases, gemstones exhibit fluorescence, which means they emit light at
specific wavelengths when stimulated by incident light. The resulting view
through a spectroscope reveals emission lines, forming the emission spectrum.
Ruby & Red spinel are notable examples of gemstones that exhibit this
phenomenon. They display emission lines at the red end of the spectrum caused by
chromium. Red spinel's emission lines are often described as "organ pipes."
Observing emission lines is optimal when the specimen is illuminated with blue-
filtered light.
Spectroscopy is the field of
study that measures and
interprets the electromagnetic
spectra that result from
the interaction between EMR &
matter as a function of the
wavelength or frequency of
the radiation. In simpler
terms, spectroscopy is the
precise study of color as
generalized from visible
light to all bands of
the electromagnetic spectrum.

Types of
Spectroscope:
Diffraction grating spectroscope
The diffraction grating spectroscope is based on the principle
of diffraction. Maybe the best-known brand is OPL, which is
produced in the UK by Colin Winter.
Light enters through a narrow slit and is then diffracted by a
thin film of diffraction grating material. This produces a linear
spectrum image with a generally larger view of the red part
than a prism spectroscope.
These spectroscopes do not have a built-in scale.
In gemology, we make use of two
different types of spectroscopes,
each with its own characteristics.
1. Diffraction grating
Spectroscope
2. Prism Spectrscope
20XX 66
Diffraction grating spectra :
a) Without scale
b) With scale in nm
a)
b)
Basic Schematic

Prism spectroscope
The prism spectroscope is based on dispersion. The light
enters through a narrow slit (some models allow you to adjust
the width of the slit) and is then dispersed through a series of
prisms. Some models have an attachment with a built-in scale.
These models are generally more expensive than their
diffraction type cousins.
Because prism spectroscopes are based on dispersion, the
blue area of the spectrum is more spread out and the red
parts are more condensed than the diffraction grating types.
Prism Spectra : Without scale and with scale in nm.
Basic Schematic

To use a spectroscope, it is recommended to begin
by testing it against various light sources to identify
absorption bands. Proper use of the instrument
and lighting is crucial to obtain accurate spectra of
gemstones. The most common technique is
reflecting light off the gemstone at a 45-degree
angle.
To avoid false readings, use a black, non-reflective
surface underneath the stone. Another technique is
holding the gemstone and light source in one hand
to view it in transmitted light. Experience can
eliminate the need for a spectroscope stand.
Using a Spectroscope:

20XX 69
A selection of principal gemstone spectra as seen using a prism
spectroscope

• Certain gemstones, such as apatite, YAG (yttrium aluminum
garnet), colored strontium titanate, and some cubic
zirconium oxide (CZ), exhibit a 'fine line' spectrum.
• This spectrum consists of thin absorption lines distributed
across the spectrum, caused by the presence of rare earth
elements like neodymium and praseodymium.
• While zircons may also show a similar spectrum due to trace
uranium, the absorption lines in their spectrum are more
accurately described as bands.
• Identification of Synthetic Gems:
• Man-made gems that owe their color to rare earth elements
display a distinct fine line spectrum. This spectrum is notably
weaker in natural gems like apatite.
• A strong fine line spectrum is a positive indication of a
synthetic gem. Unlike transition elements, rare earth
elements produce unique patterns of fine lines irrespective
of the host crystal.
1. The upper spectrum is produced by the rare earth Erbium and can be seen in
pink CZ and in pink rare earth garnets (YAG).
2. The lower spectrum is caused by dysprosium in yellow/green rare earth
garnets

Fraunhofer, a Bavarian scientist, detailed the solar spectrum, revealing fine lines known as Fraunhofer lines.
Gemmologists often use the Fraunhofer B and G lines for measuring gem dispersion. The B line is at 686.7 nm in the
red, and the G line is at 430.8 nm in the blue. These lines serve as standard wavelengths for dispersion measurement in
gemmology.
For example, when testing a
gemstone, if you observe absorption
lines in the blue region at 450, 460
and 470 nanometers (nm), it
indicates an iron-caused visible light
spectrum. The conclusion is that the
gem is a natural sapphire.
Fraunhofer’s drawings of the solar spectrum

X-Ray Diffraction
Studies:
X-ray diffraction is a scientific
technique that analyzes the
crystallographic structure of a
material. It measures the diffraction
patterns generated when X-rays
interact with the crystal lattice. The
crystal structure can be determined
by measuring the angles and
intensities of the diffracted X-rays.
X-ray diffraction is a crucial tool in
gemology. It is used to identify
minerals and gemstones based on
their crystal structures, characterize
their internal arrangement of
atoms, detect changes in their
structure, and ensure the quality of
gem materials. It is also employed
in gemmological research and
mineralogical studies to explore
new gem deposits and identify
minerals in various geological
contexts.
20XX 72

• It is also possible to differentiate
based on main components, i.e.
whether a ruby or sapphire is
really corundum (Al2 O3 ) or just
colored glass (SiO2 ).
• The spectrum shows the
corundum ruby and sapphire, as
well as an emerald with clearly
different signals of the main
components Al and Si.
• The contents of detectable
elements range from about 10
ppm to a percentage range.
XRF-XRD patterns

• This graph shows a comparison
spectrum of a natural and an
artificial ruby.
• Traces of additional elements such
as gallium and bismuth are
detectable in natural gemstones,
which are not present in synthetic
gemstones. These are spectrally
purer, so they contain hardly any
additional trace elements.

• Artificial gemstones can be distinguished from
natural gemstones based on their coloring
elements.
• In this spectrum, the blue color of the synthetic
sapphire has been created by adding cobalt
oxide.
• However, no cobalt oxide is present in the
natural gemstone. Instead, the color originates
from iron oxide.

Pearls can be cultured, but a saltwater pearl is much more
valuable than a freshwater pearl due to the effort required to
cultivate it, and it looks very similar.
Determining whether it is a
fresh or salt water pearl can be
done with an XRF instrument by
analyzing the manganese
content, because the matrix in
both kinds is calcium carbonate
(CaCO3 ) and thus identical. If a
pearl is cultured in fresh water,
more manganese is stored in it
than in a saltwater pearl. Fresh
water contains a relatively high
amount of manganese (5 ppb)
compared to salt water

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