by Erum Qureshi
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.
(Image courtesy: Orbital Joe)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.
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.
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.
Gemstones of the World by Walter Schumann
Gems by R Webster