Optical properties of gems

1. OPTICAL PROPERTIES

2. OPTICAL PROPERTIES:
Colour Transparency
Refractive index
Anisotropism
Dispersion
Reflectivity
Absorption
Pleochroism
Special optical effect
Luminescence

3. Introduction:
Properties of light:
Like heat, light is a form of radiant energy. Light
travels in straight line- its velocity in vacuum is 3,00,000
km/sec.
Maxwell wave theory and planks quantum theory wave
theory:
Light is electromagnetic in nature, generates in the
form of waves having undulating motion. It consists of
rapidily alternating electric and magnetic field, at right
angles to each other in the direction of propagation.
Quantum theory:
Energy is radiated in invisible particles called
‘Quanta’ whose size is controlled by the frequency of the
radiation.
Wave theory explains the phenomenon of reflection,
refraction, interference, polarization and diffraction more
satisfactorily.

4. Distance between any two successive identical points such
as crests or troughs of the wave of the light- Wave length.
Maximum displacement on either side in the direction of
motion – Amplitude.
Number of waves encountered per second- Frequency.
Frequency X Wave length = Velocity
In transparent substances- light does not just pass in
between atoms- but sets the atoms to vibrate and it is
further passed on to next atom. Finally it emerges at the
other end.
Path and velocity of light is modified- when it passes from
one medium to other
This behavior of light has great significance in study of
minerals.

5. Colour transparency:
Majority of precious stones are transparent.
Uncut stones- light passage is obstructed by rough and
uneven faces.
Cut and polished stones are clear and transparent.
Greater the transparency, more highly priced eg.
Diamond, ruby and sapphire.
Less cost- Transparent stones include- rock crystal and
amethyst
Non- transparent- opal and turquoise- value high
Whereas agate, malachite are of less value- non-
transparent stones.
A perfectly water- clear stone with – highest degree of
transparent and free from trace of colour- FIRST OF
PUREST WATER.
A stone with complete absence of colour- with perfect
transparency like diamond are WATER CLEAR OR LIMPID

6. Cloudy stones with tinge of colour- stone of second water, further
imperfections- stones of third water.
Partial passage of light- semi transparent.
Flame viewed through such mineral- will not be distinct in outline
eg. Chalcedony.
A mineral or stone through which some of light of a flame can pass-
blurred outline not at all seen- TRANSPARENT. Eg. Opal.
Complete cut off all light- Opaque eg. Hematite – but luster and
colour has made variety of gem.
Presence of cracks and fissures or foreign matter- reduces colour
transparency.
Transparent stones with cloudiness due to extremely minute
cavities arranged in strings- gives silky shine or glimmer jewellers
describe SILKY.
Transparency of a mineral is largely dependent upon its structure.
Aggregates of small crystals in compact way makes opaque .
Presence of fibers or scales- certain amount of light is lost by
scattering. Eg. Chalcedony, Chrysoprase.

7. Visible spectrum:
Electromagnetic spectrum is composed of several components
whose wave length, frequency and quanta of energy vary from one
component to other.
At one end of spectrum, cosmic rays have extremely short
wavelengths followed by X-Rays, γ- Rays and Ultraviolet- Rays.
The other end is characterized by radio waves with large with wave
lengths.
Visible spectrum has wavelengths between 390nm and 780nm
The region beyond 780nm (eg. Infrared rays) and 390nm (Ultraviolet
rays) are invisible to human eye.
Colour modification in some gem species require exposure to high
energy radiation such as γ- Rays which extensively induce colour
centers.

9. Light reflection and refraction:
When light enters from one medium to another, a part of it is
reflected back into the first medium which the other part passes
into the second medium. These two properties are referred to as
Reflection and Refraction respectively.

10. In addition, incident ray, reflected ray lie in same plane as normal
to the surface between two media (N,X, X’) refracted ray that
passes as to its second medium abruptly changes its path. The
refracted ray that enters from a rarer medium to a denser medium
bends towards the normal while the ray entering from denser to a
rarer medium bends away from the normal.
Angle of incidence AON ≠ Angle of Reflection N’OC

11. Ratio exists between them- depends on nature of two media
and wavelength of light.
This is known as law of refraction or Snells law (W. Snell,
1621).
Ratio of sine of angle of incidence and the angle of refraction.
The law of refraction also states that the incident and refracted
rays lie in the same plane as the normal drawn to the contact
(N,N’,X,X’). apart from refraction, the wavelength and
velocity of light in modified when it enters a different medium.
The frequency however, always remain constant.

12. Refractive Index:
Light ray passing from one medium to another is bent or
refracted and wavelength and velocity are changed. This varies
from one substance to another, which is controlled by type of
atom present and their packing.
Refractive index=
RI= n=

13. RI= Velocity of light in air/vacuum
This property is very helpful in identification of minerals.
Quality of reflection from a smooth surface of mineral
(Lustre) depends on RI.
Higher the RI brighter is its luster.
Customarily the RI of a substance is calculated for the
monochromatic yellow (Sodium) Light (589.3 nm).
Eg. RI of Diamond and Fluorite- which occupy two extreme
ranged gem minerals.

14. In diamond, the light ray (589.3 nm) striking from air at
an angle of 40° refracts to an angle of 15° 24', where as
fluorite it reflects to an angle of 26°42'. Thus RI for
Diamond is sin40°÷ sin15° 24'= 2.42
For fluorite sin40°÷sin26°42' = 1.43
Another way of calculating RI is taking the ratio of
velocity of light in air and the substance
RI=

15. 1.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.

16. The RI of a gemstone/gem is determined by Refractometer). 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). In reality, however, a contact fluid
(saturated solution of sulphur in di-iodomethane and
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:

17. (i) As light converge from the prism
onto the surface of the gem (Fig 1),
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.

18. 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:

19. 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.

20. Critical angle:
The light traveling from a denser medium to a rarer medium
refracts away from the normal.
However, the light ray striking the contact at a particular
angle is not passed on to the rarer, but it grazes the contact
between the two media. This angle is known as the critical
angle.
Beyond this range the rays striking at any angle is reflected
back into the original medium with full intensity. This
phenomenon is described as the total reflection and CA as the
critical angle of total reflection.
In diamond CA is as low as 24°24'. Thus RI of Diamond (n= sini ÷
sinr)=sin90° ÷ sin24°24' i.e. 1.0÷0.4131=2.42
The fluorite CA is as much as 44°22' therefore RI=1.43.
The CA given above is for the yellow light (WL 589.3 nm). For
other wavelength the CA varies and therefore the RI.

21. The basic construction of the critical
angle refractometer is shown in Fig 2
(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.
Fig.2

22. RI= Velocity of light in air/vacuum
This property is very helpful in identification of minerals.
Quality of reflection from a smooth surface of mineral (Lustre)
depends on RI.
Higher the RI brighter is its luster.
Customarily the RI of a substance is calculated for the
monochromatic yellow (Sodium) Light (589.3 nm).
Eg. RI of Diamond and Fluorite- which occupy two extreme ranged
gem minerals.
In diamond, the light ray (589.3 nm) striking from air at an angle of
40° refracts to an angle of 15° 24', where as fluorite it reflects to an
angle of 26°42'. Thus RI for Diamond is sin40°÷ sin15° 24'= 2.42
For fluorite sin40°÷sin26°42' = 1.43

24. Refractometer:
Based on CA and total reflection, a device
known as the refractometer is constructed
to determine the RI of substance
prerequisite for this experiment is that the
substance should have a smooth surface
without irregularities.
There are several versions of
refractometers.
Most common ones used have a range up
to 1.81.
Back bone of the refractometer is the
hemisphere of dense glass.
Containing lead oxide (RI 1.86 to 1.90) with
its flat polished surface exposed on the
platform.The light (589.3 nm is taken in
practice) is made to pass through a ground
glass which falls on a quadrant of
hemisphere.Before placing the stone a
drops of contact liquid is added so that air
gap between the interface of stone and

25. The contact liquid however, should have a RI
higher that that of the stone and lesser than
that of the hemisphere.
Commonly used contact fluid is methylene
chloride (RI 1.74) saturated with sulphur to
obtain a RI of 1.81.
When other liquids of lower RI are used only
the stones having RI lesser than that of liquid
can be determined.
Light rays that strike the interface of two
media (i.e. hemisphere and stone) at an angle
greater than CA of the substance (Stone) are
totally reflected back into the hemisphere.
The rays striking at angles less than CA are
refracted into the stone placed over the
hemisphere. Thus it results in a region of
brightness adjacent to a darker counterpart
representing the range of total reflection and
refraction respectively. However, the darker
region is totally dark.

27. A scale is placed in the path of the
reflected light which can be read
from an eye piece. Often function
between dark and bright regions are
referred as the shadow edge.
Instead of sodium light, if white light
is used, function between the lane
and light parts would be hazy with
colour spectral bands due to
dispersion.

28. In such case the reading can be taken at
yellow green boundary placing a deep
yellow filter over the eyepiece solve the
problem.
The faint line representing the RI for the
thin layer of contact fluid is also
produced in the scale.
RI of the stone can be calculated in the
following way sinCA = RI of rarer
medium ÷ RI of denser medium.
When fluorite is taken for
determination of RI against fluorite
placed over the hemisphere of RI 1.86
,the CA obtained is 50°15'.

29. Thus RI of fluorite would be sin50°15' X 1.86 = 1.43.
Singly refracting substance provides a single shadow edge, while
doudly refracting substance given rise is a double shadow edge.
Eg.nE
= 1.62 and nW
= 1.64 in tourmaline.
Cryptocrystalline doubly refracting substances (eg. chalcedony)
results in a single generalized, not so sharp reading.
Limitations:
Main limitation is the common refractometer is that the RI of
stones up to only 1.81 can be determined, as RI of normally used
contact fluid does not go beyond 1.01. however, most
gemstones fall within this range.
Secondly, lead glass is quite a soft material and H is liable for
scratching from harder stones, when carefully used. Scratch
marks on the hemisphere makes the refractometer useless.

30. Only a small drop of fluid has to be added and weighed carefully
by using a soft absorbent material like tissue paper.
For stones having RI greater than 1.81, RI plus refractometer
with strontium titanium prism (RI 3.41-5.5) –devised by A. Krun
optical works, germany is used.
It can cover up to range of RI 1.79 to 2.21
Diamond table refractometer, which consists of prism for
determination of RI in the range of 1.59 to 2.03.
It has the advantage of being resistant to both abrasion and
chemical attack, Wents solution (RI 2.05) is used as contact liquid.
Diamond table refractometer is manufactured by Rayner optical
company Ltd, England.
S and T electro optical systems, California has developed a
refractometer having cubic zirconium prism (RI 2.17 to 8.11) this
has a range of 1.40 to 2.10. It has a built in LED yellow light source.

31. Methods described above are useful for stones
having a flat polished surface. For determining RI of
stones cut into a rounded carbochon form, a method
known as distant vision method or spot method is
employed. A minute drop of contact liquid is applied
in the surface of hemisphere on which curved surface
of such stone is placed. The scale is read from a
distance of 30-55cm away from the eyepiece. This
method requires sufficient practice.

32. Reflectivity meter and table spectrometer:
Reflectivity meter is the recent instrument to measure RI, used
especially for diamond and its stimulants.
This instrument measures the luster or relative reflectivity of a well
polished surface and provides the percentage of light that is reflected back
when it strikes the smooth surfaces at right angles.
However, it is easier said than done to strike a surface exactly at right
angle and to measure the reflected light at the same time.
Therefore in practice, the angles of incidence and reflection are
somewhat offset from the normal (Read, 1983). The method taken with
consideration to formula.
Reflectivity= intensity of reflected ray (n-a)2
÷ intensity of incident ray
(n+a)2
Where a= RI of surrounding medium (air= 1.0)
n= RI of gem stone.
The equation is further multiplied by 100 to obtain the percentage of
light reflected back from the incident light that falls on its surface.
Eg, In diamond, 17% of light is reflected back (RI 2.42)
In fluorite, (RI 1.43), 3% of light is reflected back.

33. The light used in reflectivitymeter in infrared beam having a
wavelength of 930 nm
Available reflectivitymeter are Jewellers eye, Diamond eye and
Gemlyser. Duotester, a model of Presidium diamond PVT Ltd.
Singapore- Measure both reflectivity and thermal properties.
Table spectrometer, eg: Kruss gemstone spectroscope KL 1302
or Lang Spectrometer is specially useful in measuring the RI of the
Stone that goes beyond the range of conventional refractometer
(eg. Diamond and its modern simulants).
Table spectrometer consists of a Graduated Rotating table on
which the Stone (is the form of the prism or with suitable facts) is
mounted. It is provided with a fixed light source. Rays refracted
from the stone are viewed by a radially- mounted telescope. By
using approximate light source, monochromatic or suitable
interference filter, in precise values can be obtained.

34. Single Refraction, Double Refraction and Birefringence:
Minerals crystallize in six crystal systems. In crystals
belonging to the cubic system RI is identical in all direction. i.e. that is
they are singly refractive. Theoretically when a torch is lit at the
centre of the crystal light rays move with equal velocity in all
directions resulting spherical wave front. These are called isotropic
substances like isometric crystals, amorphous substances, liquids and
gases are also isotropic.
An object seen through a transparent calcite (Iceland spar) is
split into two images due to variation in RI in two directions right
angles to each other. Such crystals are said to be doubly refracting.
The substances crystallizing in systems other than Cubic are
characterized by double refraction. Tetragonal and hexagonal crystals
posses two distinct values of RI, one along ‘c’ axis ( ) and other alongɛ
‘a’ axis (ω); in some crystals has greater value of RI while in someɛ
others ω. Whereas orthorhombic, monoclinic and triclinic crystals
posses three distinct RI values.

35. •Maximum value in one direction, formed as γ and the ray
vibrating in this direction is slowest ( ).ƻ
•Minimum value at right angles to the former, called α and the
ray vibrating in this direction is the fastest (X).
•A value intermediate between the former two lying in a
direction perpendicular to the plane that includes X and T both,
designated as β and the velocity of the vibration direction (Y) is
intermediate between the two.

36. In doubly refracting calcite , RI in C-axis direction is 1.4864( ,ɛ
light vibrating in this direction is known as the extraordinary ray
or e-ray) and in the direction of ‘a’ axis, RI is 1.6585 (ω light
vibrating in this direction is known as ordinary ray or O- ray).
Difference between the two values (0.1721) is known as the
Birefringence.
It is also expressed as n2
-n1
, where in n2
and n1
, stand for the
values of maximum and minimum refractive indices receptive in
the crystal. The difference in RI is also abbreviated as DR or δ. The
difference in values between the maximum and minimum RI (ɛ
and ω) in tetragonal and hexagonal crystals, γ & α in
orthorhombic, monoclinic and triclinic crystals is smaller in some
crystals (eg. Apatite, quartz , and feldspar ; birefringence weak)
and greater in some other crystals (eg. Rutile and calcite;
birefringence extreme).

37. When faceted stones characterized by strong birefringence (eg.
Peridot, zircon and sphene) are viewed from the table facet, the
culet and back facets appear in two sets (doubling of the back
facets).
A 10X loupe is quite handy to observe the doubling effect. If the
table is cut perpendicular to the optic axis, the stone has to be
tilted to notice the doubling effect. The doubling effects however is
not obvious in the stones having weak birefringence (eg. apatite).

38. Polarised light:
Light produced in a luminous body travels in straight line vibrating
in all directions at right angles around the line of transmission when its
vibration is made to restrict in only one plane in the direction of
propagation, it is said to be plane polarized light.
The device that polarizes light is called a polarizer and can be
obtained by several methods, of which nicol prism and polaroid are very
effective devices used. When two such polarizers are kept with their
vibrations perpendicular to each other, the polarized light vibrating parallel
to one plane in first unit gets cancelled by the second unit which eventually
allows the light vibrating only in a direction perpendicular to the former
one.
Therefore the polarizers that are crossed at 90° position results the
darkness. This position is called the crossed polars or crossed nicols (X-n).
For certain observations only one polarizer that produces plane
polarized light is used (PPL). PPL can be obtained by one of the following
method. A) Polarization by reflection B) Nicol prisms C) Polarization by
absorption D) Polaroid as polarizing filter.

39. Polariscope and polarizing
microscope:
A simple but effective
instrument to study transparent
substances is polariscope.
Polariscope consists of pair of
polaroids. Generally the lower
polaroid or polarizer is fixed.
While the upper polaroid or
analyser is rotatable. To obtain
crossed nicol conditions (X-n) also
known as crossed polaroids or
polars, the vibration direction of
the two polaroids is kept at right
to each other. To make
observations under PPL, the
upper polaroid is removed.

40. Polarizing microscope or
petrological microscope consists of
a 1) A pair of polarisers, one below
the stage and the second
(analyser) plased between
objective and ocular 2) graduated
rotating stage. 3) A slit in a tube of
microscope to introduce optic
accessories and 4) A Bertrand lens
to enlarge interference figures.

41. Use of polariscope:
•Isotropic/anisotropic nature and anomalous
birefringence can be identified. Red spinel mistaken for
ruby can be distinguished when viewed between crossed
polaroids, isotropic red spinel remains dark where as ruby
transmits bright light (exception in extinction position and
optic axis). Strained isometric substances or glass are
slightly anisotropic.

42. •Interference figure can be
determined by super imposing a
spherical bead over an anisotropic
stone reveals interference figure. In
faceted stones, observing interface
figure in quite tricky. The stone is
gradually turned in various directions
by using tweezers till a distinct first
order interference colour is obtained
(total darkness in seldom obtained).
As soon as such orientation is
achieved, a conoscope is
superimposed just above the stone
instantaneously a centered figure
appears: a cross with colour rings if it
is uniaxial or a single isogyre with
isochromatic curves around melatope,
in the case of a biaxial stone.

43. •Pleochroism: while observing the property of pleochroism
upper polaroid or analyser is removed.
•Proficiency: to achieve this, gemologist has to familiarize with
the following examination of stones
• Polished plates (transparent) but at right angles to optic axis
of tourmaline, beryl, apatite, corundum or quartz for uniaxil
figures.
•Tabular sphene, diopside, epidote, moonstone and topaz,
muscovite mica for biaxial figures.
•Above crystals cut in cabochon shape- for observation of
interference figure.
•A transparent scolecite needle for understanding the concepts
of twining and extinction angle.

44. Dichroscope:
Dichroscope is used for finding selective absorption of
colours in anisotropic substances (dichroism/ pleochroism).
It is simple instrument consisting of a cleavage rhomb of
calcite glued with glass prism on either end.
The unit is fitted into a metal tube whose one end has a
bare window or aperture for placing the crystal under
observation. The other end is provided with an eye piece.

45. When a pleochroic crystal is viewed (in other that optic axis
direction) two images with different shades of colour are
obtained. However if a polariscope is already available,
dichroscope becomes superfluous. In an polariscope the
pleochroic colour can be noted by using only the lower
polaroid.

46. One more method of observing pleochroic colours is to use two
four pieces of polaroid sheets placed in the position in stone
placed at the function of sheets exhibits different shades of
colour.

48. Optic axis:
An anisotropic crystal is characterized by varying RI in
different directions. There are however, certain sections in which the
RI is identical in all directions. The axis perpendicular to this section
is called the optic axis. Tetragonal and hexagonal crystals posses only
one optic axis, while orthorhombic, monoclinic and triclinic crystals
of two such optic axis. Crystals which posses only me opticaxis are
uniaxial and later biaxial.

49. Optic sign:
Velocity of e-ray that vibrates parallel to C-axis is faster in
some uniaxial crystals while in others it is slower when compared
to that of O-ray. If e-ray is faster. It is said to have negative optic
sign or optically negative and when it is slower, the crystal is said
to be optically positive.
In other words, when inversely proportional values of RI
are taken into consideration; if the value of is less than ω, theɛ
crystal is optically negative and when is greater than ω, it isɛ
optically positive.
For eg. In calcite = 1.4864 and ω= 1.6585 and hence its opticɛ
sign is negative. In quartz = 1.5533 and ω= 1.5442 and hence itsɛ
optic sign is positive.

50. Interference figure: is obtained in convergent polarized light or conoscopic
condition. Helps in knowing isotropic, uniaxial and biaxial nature of crystals.
Extinction:
Convergent light and conoscope: light strikes a crystal from various direction
converging at a point in the form of a cone is known as convergent light
orconoscopic conditions.
Interference colour: when polarized light enters an anisotropic substance it
splits into two rays vibrating at right angles to each other one vibrating
slowly in the direction of higher RI and other vibrating more rapidly along
the direction of lower RI. In other words, when the two rays leave an
anisotropic substance, the ray vibrating in one direction is retarded with
reference to the ray vibrating at right angles to it. This kind of movement
results in a path difference for the two bundles of polarized light and they
are made to pass through upper polarized or analyzer whose vibration is
placed at right angles to each other having a certain path difference enter
analyzer, they are resolved into the vibration direction of analyzer and they
interfere with each other. In this program some components are enhanced
and some are extinguished.