Chinese Jade

CHINESE JADE

BY

FRANK DAVIS

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Introduction to Gemmology

Gemmology is the science dealing with natural and artificial gems and gemstones. It is considered a geoscience and a branch of mineralogy. Although some practice gemmology as a sole profession, often jewellers become academically trained gemmologists, qualified to identify and evaluate gems. Rudimentary education in gemmology for jewellers and gemmologists began in the nineteenth century, but the first qualifications were instigated after the ‘National Association of Goldsmiths of Great Britain’ (NAG), set up a Gemmological Committee for this purpose in 1908. This committee matured into the ‘Gemmological Association of Great Britain’ (also known as Gem-A), now an educational charity and accredited awarding body, with its courses taught worldwide. The first US graduate of Gem-A's Diploma Course, in 1929, was Robert Shipley who later established both the ‘Gemmological Institute of America’ and the ‘American Gem Society’. There are now several professional schools and associations of gemmologists and certification programs around the world.

The first gemmological laboratory serving the jewellery trade was established in London in 1925, prompted by the influx of the newly developed 'cultured pearl' and advances in the synthesis of rubies and sapphires. There are now numerous Gem Labs around the world requiring ever more advanced equipment and experience to identify the new challenges - such as treatments to gems, new synthetics and other new materials. Gemstones are basically categorized based on of their crystal structure, specific gravity, refractive index and other optical properties such as pleochroism. The physical property of ‘hardness’ is defined by the nonlinear ‘Mohs Scale’ of mineral hardness. Gemmologists study these factors while valuing or appraising cut and polished gemstones. Gemmological microscopic study of the internal structure is used to determine whether a gem is synthetic or natural, by revealing natural fluid inclusions and partially melted exogenous crystals, in order to demonstrate evidence of heat treatment to enhance colour. The spectroscopic analysis of cut gemstones also allows a gemmologist to understand the atomic structure and identify its origin; a major factor in valuing a gemstone. For example, a ruby from Burma will have definite internal and optical activity variance as compared to a Thai ruby.

Gem identification is basically a process of elimination. Gemstones of similar colour undergo nondestructive optical testing until there is only one possible identity. Any single test is indicative, only. For example, the specific gravity of ruby is 4.00, glass is 3.15-4.20, and cubic zirconia is 5.6-5.9. So, one can easily tell the difference between cubic zirconia and the other two; however, there is overlap between ruby and glass. And, as with all naturally occurring materials, no two gems are identical. The geological environment in which they are created influences the overall process, so that although the basics can be identified, the presence of chemical ‘impurities’ and substitutions along with structural imperfections vary - thus creating ‘individuals.’ Having said this, the three main methods of testing gems are highly successful in proper identification. These are:

• Identification by refractive index - This test determines the gems identity by measuring the refraction of light in the gem. Every material has a critical angle, at which point light is reflected back internally. This can be measured and thus used to determine the gem’s identity. Typically, this is measured using a refractometer, although it is possible to measure it using a microscope.

• Identification by specific gravity – This method, also known as ‘relative density’, varies depending upon the chemical composition and crystal structure type. Heavy liquids with a known specific gravity are used to test loose gemstones. Specific gravity is measured by comparing the weight of the gem in air with the weight of the gem suspended in water.

• Identification by spectroscopy – This technique uses a similar principle to how a prism works, to separate white light into its component colours. A gemmological spectroscope is utilised to analyse the selective absorption of light in the gem material. Essentially, when light passes from one medium to another, it bends. Blue light bends more than red light. Depending on the gem material, it will adjust how much this light bends. Colouring agents or chromophores show bands in the spectroscope and indicate which element is responsible for the gem's colour.

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