How to identify
the Emerald GemstoneBy: Erum Qureshi
Summary: Step by step lab procedure to identify the green stone emerald in scientific manner. Learn from an expert, how the Refractive Index, Double Refraction, Absorption Spectra is used to determine the stone type.
Emeralds belong to the
species Beryl
 The property that distinguishes
an Emerald from other Beryls is
the characteristic deep green or
bluish green color caused by the
presence of chromium. All Beryls
of other colors of gemstone
quality are called precious
beryl and not emeralds.The first thing that a trained
gemologist looks for is the
color.
If the color of the stone is
deep green (Emerald green),
green or even slightly
yellowish-green, then tests will
be conducted to specifically
identify it as an Emerald.
Step 1:
Determination of Density
(Specific Gravity or S.G):
Specific gravity indicates the
ratio of the weight of a
specific material to the weight
of the same volume of water.
It is also expressed as density
and is measured in grams per
cubic centimeter.Although a determination of the
density is useful, specialists
rely on optical procedures for
taking the final call on the
determination of the gemstone.
The density of the stone is
calculated by placing the gem in
a beaker filled with heavy
liquid or high-density liquid.
This method works on the idea
that an object will float in a
liquid of higher density, sink
in a liquid of lower density and
remain suspended in a liquid of
the same density. Sets of
high-density liquids are sold
commercially, but need to be
handled with care due to their
toxic nature. Since the density
of emeralds is between 2.67 to
2.78 if the stone is an emerald
it will float in a liquid of
density 3.20 and remain
suspended in a liquid of density
2.67.
Step 2: Refractive Index
(R.I)
Refraction of light occurs when
light rays leave one medium and
enter obliquely into another at
the interface between the two
media.The amount of refraction (called
the refractive index) in the
crystals is specific for each
gemstone.
It can therefore be used in the
identification of the type of
stone.
The light refraction is measured
in a
Refractometer by placing
the gem face down in the
apparatus with a small amount of
contact liquid (Liquid available
in the market for gem testing).
The values can be read from the
Refractometer scale, however
testing is only possible up to a
value of 1.41 to 1.81.
For cabochons (gemstones cut
with an even stone surface and
no facets, a cut synonymous for
opaque gemstones) spot readings
can be taken if the stone is
transparent to semi-translucent.
If the purported stone IS an
emerald the reading will
fluctuate between 1.56 and 1.60.
Stones other than emeralds will
exhibit different R.I. For e.g:
the green andradite garnet (also
called Demantoid and commonly
confused with emerald) has an
R.I of 1.88 and therefore will
not show a reading at all in the
Refractometer (since a
Refractometer will only take
readings up to 1.81).
Tourmalines, also commonly
confused with emeralds have an
R.I between 1.61 and 1.66, which
will again indicate that the
said stone is NOT an emerald.
Determination of the R.I however
will not help in distinguishing
between natural and synthetic
emeralds.
Step 3: Double
Refraction (D.R)
Just as a ray of light is
refracted when entering the
crystal of a gemstone, it is at
the same time divided into two
rays.
This phenomenon is known as
double refraction. It is
expressed as the difference
between the highest and lowest
refractive index and can be
useful in identifying gemstones.
Some gemstones are singly
refractive and some doubly
refractive and to determine
this, a polariscope is used.Emeralds are doubly refractive
gems therefore, when viewed in a
polariscope, show the
characteristic DR effect, which
is alternate dark and light
effects when the stone is
rotated 360 degrees.
Step 4: Pleochroism
When viewed in different
directions some gems appear to
have different colors or depth
of color. This is caused by the
differing absorption of light
rays in double refractive
crystals. Where two main colors
can be observed (tetragonal,
hexagonal and Trigonal crystal
systems) we speak of Dichroism
or
Pleochroism (a descriptive
term for multi-coloredness)The instrument for observing
Pleochroism is the
dichroscope.
The crystal structure of
emeralds is Hexagonal /
hexagonal prisms and when viewed
through the dichroscope it will
appear Dichroic (that is two
definite colors can be observed-
distinct green, blue-green to
yellow-green).
Step 5: Absorption Spectra
The absorption spectrum of a
stone consists of the bands or
lines that appear in the
spectral colors of light as they
emerge from the gemstone.
The instrument used to determine
the wavelength of the absorbed
light is the
spectroscope.
The wavelength is measured in
nanometers (symbol nm) or
Angstrom symbol A.
Like most gems Emeralds have a very
characteristic, almost unique
absorption spectrum. When viewed
through the spectroscope the
absorption spectrum of an emerald
will show this characteristic black
vertical lines or bands.
The absorption spectra of
natural emerald (in nanometers):
683, 681, 662, 646, 637,
(606), (594), 630-580, 477, 472.
The absorption spectra of
synthetic emerald: 683,
680,
662, 646, 637, 630-580, 606,
594, 477, 472, 430.
(Strong absorption lines are in
bold letters; weak ones are in
parenthesis.)
This characteristic
absorption spectrum determines
without any doubt the stone or
glass used to imitate the
emerald. By this method one can
easily differentiate between
gems of the same density and
similar refractive index.
Rough stones, cabochons and even
set stones can be effectively
tested by this method.
Microscopic Examination
 This is undoubtedly the most
important and deciding test in
determining the authenticity of
any gemstone. Gemstones contain
foreign matter, or some kind of
dislocation or irregularity in
the crystal lattice. These are
visible to the naked eye or
under magnification and are
known as inclusions.Inclusions are not accidental
but are subject to strict
conformities with natural law,
they can tell a lot about the
origin of the gemstone and also
help in identification.
The final step to identifying
any gemstone is the physical
examination of the specimen with
a 10 X loupe and / or a
microscope. (Image courtesy:
Orbital Joe)
There are forms of inclusions
specific to every gemstone as
also to its imitation. Natural
emeralds when examined under a
microscope show two phase
(liquid and gaseous) or
three-phase (crystal, liquid and
gaseous) inclusions. These
appear as tiny black inclusions,
these are embedded in the
crystal during the formation of
the mineral. Some other
inclusions are needles and
crystals. Some typical
inclusions are characteristic to
the place of origin of the
emerald.
For example tremolite needles
and mica plates with
well-rounded edges in Russian
emeralds, typical three-phase
and inclusions of well formed
pyrite crystals, actinolite and
comma-shaped inclusions in
Indian emeralds.
Synthetic emeralds under high
magnification will show gas
bubbles (which appear as soap
water bubbles) and traces of
unmelted powder and swirls as
inclusions.
Fluorescence
Another test to determine
synthetics from natural emeralds
is exposing it to Ultraviolet
radiation (or UV rays).
Synthetic emeralds transmit UV
light more than natural
emeralds.There are two common
ultra-violet rays used for
fluorescence. Those are short
wave and long wave rays.
Short-wave rays are ones with
smaller wavelengths
but with lighter frequencies and
power in terms of penetration to
the gemstone. Exposed to
short-wave rays emeralds may
show reddish fluorescence and
when exposed to long-wave rays
neutral fluorescence.
Alternatively the Chelsea Filter
(an apparatus used by
gemologists through which when
an Emerald is viewed it appears
red due to its chrome content)
may be used to determine
specific red fluorescence under
short wave.
Consumers should
also beware of fluorite and chrome
Diopside, which are very close
imitations of emeralds when set
in jewellery.
References:
1. The Chelsea Filter
image courtesy:
mineralab.com
2.
Gemstones of the World by Walter
Schumann
3.
Gems by R Webster
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