Gemstone Testing tools
by Ms Nandita Ray
It is tricky to test a gemstone. Some stones are soft, some are mounted while some can be coated, treated or easily scratched. One has to be very careful so as not to damage the stone in any way. With technology advancing rapidly, the tools used for testing are made to suit ones needs and pockets. Aiding the tools is a wide range of database, which lists the particular characteristics of gems.
Tools Required to Test Gemstones
There is an absolute and definite need to test gemstones, especially if they are expensive. Competition, economics, availability and demand have all worked towards the explosion of the gem market. The markets are flooded with gemstones of various hues, brand and breed. Therefore the need to test the authenticity of gemstones is imperative. Insurance too has a hand. They need the gem to be certified.
In the past, it was pretty easy for a trained gemologist to distinguish a flawless gem from a flawed one. There were no synthetic gems during those days. Today however, one has to determine whether a gem is real or not. Trained eyes cannot always avoid error, particularly, if the gem has been manufactured in the laboratory. It becomes a challenging task. Earlier on a magnifying glass was good enough. Presently however, a host of instruments are used, especially if one wants to be absolutely certain of the gem, its history and pedigree.
It is tricky to test a gemstone. Some stones are soft, some are mounted while some can be coated, treated or easily scratched. One has to be very careful so as not to damage the stone in any way. With technology advancing rapidly, the tools used for testing are made to suit ones needs and pockets. Aiding the tools is a wide range of database, which lists the particular characteristics of gems. This is valuable because the markets are deluged with zircons or the synthetic Moissanite, which are look alike diamonds and very difficult to differentiate from a real diamond.
Loupe is actually a sophisticated magnifying glass. It magnifies the gemstone in detail. The internal flaws, surface blemishes, cracks and any other cover ups are easily seen. It is a powerful convex lens that produces a magnified image of the gemstone. The standard loupe that used is 10X, which magnifies the gem ten times its size. This standard is mandatory for testing gemstones in the USA.
Binocular Microscope is equipped to spread light from the bottom and from the sides. Its function is similar to the loupe. Presence of the lights and its superior optical strength provides excellent clarity. It usually has 10X magnification power though 200X can be got. Most common ones are 10X to 40X. There are stages of adjusting the iris diaphragm. It also provides coarse to fine focusing.
Refractometer measures the refractive index of the gemstone. This is needed in order to find out its composition and physical properties. It measures the extent to which the light that passes through the gemstone from the air is bent. It helps identify the gem by comparing the refractive index thus got with the established values. It assesses the purity of the gem by comparing it to the value of a natural gemstone. It also determines the amount of solute that has been added to the gemstone by comparing the refractive index thus obtained with the standard curve that is established. This is done by putting a drop of a liquid with a high RI, on a glass disc which is based on the tip of the Refractometer. Natural or white light is passed through this. The RI is reflected in the magnified gauge.
Dichroscope is used for the quick identification of gemstones. It can effortlessly separate gemstones based on dichroism, which is the property of a gemstone to show two contrasting colors when turned in two different directions, under a light. This is used to distinguish two physically similar looking gemstones. The dichroscope is used to view the dissimilar color tones of the dichroic piece. It can easily differentiate a Ruby from a Tourmaline, which is dichroic while a Ruby is pleochroic. Some gemstones that are dichroic are Sapphire, Topaz and Zircon, to name a few.
Polariscope is used to determine if a gemstone is natural or not. It can differentiate diamonds from Moissanite. It also detects strains in diamonds. It can differentiate between gems that have double or single refraction i.e. birefringence. Double refraction is the breaking up of a ray of light into two rays, the ordinary ray and the extraordinary ray, when it passes through some materials like calcite also called Iceland spar. This effect can take place in gems that are anisotropic in structure i.e. having properties that differ according to the direction of measurement. The polariscope helps to track and see the path the rays of light are taking through the stone that is being tested. By measuring the optic characteristics of transparent to translucent gem material, the refractive index is found. Double refractive is only a property of solids. For transparent materials, the graph of index, unlike the wave length, is curved with a few general characteristics. A gem is held between two polarized plates and the light transmitted through it is studied and based on the readings the identification is made.
Some gemstones with double refraction are Quartz, Tourmaline, Ruby, Peridot, Sapphire, Zircon and Rulite to name a few. Diamonds are isotropic, meaning single refraction. Topaz, Mica and Perovskite have triple refraction.
Ultra Violet Light source: – some gemstones start emitting a fluorescent glow after being exposed to short or long ultra violet radiation. Many gems can be easily recognized by the Fluorescent color and the locale. Some fluorescent minerals are Agrellite, Tugtupite, dolomite, Fluorite and Benotoite.
Color Filter is also called a ‘Chelsea Filter’ or ‘Emerald filter’. This instrument helps in separating natural, synthetic and imitation gem material. It helps in establishing whether similar looking stones exhibiting the same colors are natural or not. It comes handy while dealing with some green, red or blue stones, like blue Spinel and blue Topaz. It detects dyes in gemstones also.
Spectroscope is also used to separate natural gems from synthetic gemstones. This is used in specialized gem testing laboratories. The differences in the chemical composition are revealed by the absorption spectrum of the light transmitted through the gemstone that is being tested. The instrument breaks up the light that is being transmitted from a gemstone into its spectral colors. This helps in testing the various wavelengths that are being taken in i.e. being absorbed by the gemstone. This indicates the type of elements that are present in the stone, which are absorbing the wavelengths. It allows the gemologist to see which color of light is being absorbed and thus identify the gemstone.
Specific gravity Liquids
Specific gravity Liquids – this is important for identifying a number of gemstones. A carat scale also called hydrostatic balance is used.
X-Rays are sometimes conducted to separate natural pearls from cultured pearls.
Diamond Tester is used for testing diamonds. The unit is switched on and the diamond that is to be tested is touched by a pointed tip. A light will come on to indicate if the stone is a genuine diamond or not. It uses thermal conductivity to differentiate diamond from its many stimulants. When the tip is touched to the diamond, the meter will show green if it is a real diamond if not then it will remain in the red zone. This instrument can be used to test the minutest diamond, as small as 0.02 carats. It can also separate colored stones from one another. By merely touching the thermo electric probe to the gemstone, the relative heat conductivity of the material is exhibited on the dial.
These are the basic instruments. There are improved and sophisticated versions of these instruments available in the markets.