Traditional gemmological tools and methods play an important role in determining the nature of mineral species, which mainly involve determining the physical and optical constants and comparing with tables of their standards, available in common gemmological texts. These mainly provide qualitative information but still considered as important tools for gem identification.
A polariscope is an instrument which consists of two polaroid plates mounted apart. The lower plate is generally fixed and is known as the polariser, while the upper plate can be rotated and is known as the analyser.
It is used to observe the optic character i.e. whether the stone is singly refractive (isotropic) or doubly refractive (anisotropic), to locate the optic figures (uniaxial or biaxial), to orient the optic axis direction and to detect pleochroism. In some light coloured sapphires it also helps to separate natural and synthetic counterparts.
A dichroscope consists of a cleaved rhomb of optical quality calcite (Iceland spar), which is mounted in a tube having an eye piece at one end and a square/round aperture at the other. When white light passes through a doubly refractive stone, the two polarized rays emerging out of it are differently coloured i.e. they absorb different wavelengths. These two polarized rays enter into dichroscope aperture and coincided with the two polarized rays of the calcite rhomb in the dichroscope. This makes both colour components to be seen side by side.
It is used to observe pleochroism i.e. Dichroism or Trichroism. In some cases, by observing pleochroism it is possible to identify a gemstone like iolite or tanzanite and also to detect some treatments like heating in tanzanite or sapphire. However, in order to observe pleochroism, the stone must be coloured, doubly refractive, transparent to semi-transparent and a single crystal.
This instrument is based on the principle of ‘total internal reflection’ and basically measures the refractive index of a stone. Refractive index (RI) of a material is the ratio of the velocity of light in air to the velocity of light in the substance. The refractive index can be taken on any stone that has flat or curved surfaces as long as the surface is polished. A standard gemmological refractometer has an upper limit of 1.80 and hence stones with a refractive index higher than this cannot be measured.
On the basis of Refractive Index measurements, it is possible to determine whether the stone is isotropic or anisotropic, along with determination of birefringence. It also determines optic character (uniaxial or biaxial) and optic sign (positive or negative)
Gemmological spectroscope separates white light into its component spectral colours ranging from 400 to 700nm. When a gem under test is illuminated properly with white light and observed through a spectroscope, chemical ingredients present in it absorb some wavelengths while transmitting the rest; the wavelengths which are absorbed by the gem appear as dark lines or bands on the spectrum. Such spectral pattern is specific for certain types of stones and help to identify and separate from lookalikes.
Absorption spectrum gives an indication of the colouring elements present in the gemstone, thereby assisting in its identification. In some cases spectral pattern helps to separate natural gemstones from their synthetic and treated (dyed) counterparts
ULTRA VIOLET (UV) LAMP
Ultra violet radiation has a range of wavelengths from 400nm to up to 10nm. However, the usable range is from 400nm to 200 nm, where long wave varies from 410 to 300nm and short wave varies from 300 to 200nm.
UV lamp is used to observe fluorescence and phosphorescence, which helps to distinguish between natural and synthetic or treated counterparts. For example, presence of chalky fluorescence in blue sapphires is a strong indication of it being heated or a patchy orange reaction in a ruby suggests the presence of dye. However, this is not a confirmatory test and is considered to be an indicative only
Specific Gravity (SG) is defined as the weight of a body compared with the weight of an equal volume of pure water ideally at 4oC. The specific gravity together with other measurements can help in narrowing down the range of possibilities in identification. There are two methods by which the Specific Gravity of a gem material may be estimated - hydrostatic method and heavy liquid method. At GTL, hydrostatic method is employed where SG is calculated by using electronic scales which accurately measure the weight of a stone both in air (A) and in water (B)
Occasionally, reactions of a stone are also seen under a hand magnet, alcohol-swab*, pin indentations*, hardness pencils*, or a Chelsea filter.
* with permission from the depositor