Don’t trust the jeweller who claims to be able to differentiate between a genuine and synthetic gemstone with his naked eye. Even with the aid of a loupe, “it is hard to differentiate between two identical-looking gemstones or determine if they are real or synthetic,” says Isabelle Delahaye, a professor at L’École, the school of jewellery arts founded by Van Cleef & Arpels. An art historian by training, she has been working closely with the Van Cleef & Arpels’ Mains d’Or (master craftsmen) for some 11 years and is particularly well-versed in gemmology. Her “Recognise the Gemstones” class that we are enrolled in is one of 20 courses offered as part of L’École’s curriculum.
According to Delahaye, even the most seasoned of gemmologists will still find a trip to the world’s most famous gemstone markets a daunting experience. Crammed with gemstones, mine-to-market industries in cities like Chanthaburi (Thailand), Bogota (Colombia), Mogok (Myanmar) and Ratnapura (Sri Lanka) are also crowded with crafty merchants, traders and salesmen who have no qualms making a quick dishonest buck.
Relying on one’s good eye is the first but certainly not the only barometer of truth or quality. Imitation stones (mostly made of glass) sparkle but are devoid of the mineral characteristics that make up a genuine gemstone. Synthetic, or lab-grown, gemstones on the other hand, have the same mineral structure as the natural stone they are modelled after and need to be put through more sophisticated methods of testing.
The class is small and can accommodate no more than 10 students. Given a set of equipment to use — Tweezers, carat scale, loupe and other scientific apparatus — each student is also entrusted with a set of five identically coloured stones that need to be classified (to determine if it is precious, fine, synthetic or imitation) and later identified — right down to a science — at the end of the four-hour class.
As one of the most important tools used in gemmology, a polariscope (shown below) can only be used on transparent or translucent stones. It gauges a stone’s reaction to light and thus exposes its crystalline system: Triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic. A cubic crystalline structure for example, would narrow the stone down to diamond, pyrite, fluorite or glass.
Arguably the most important tool to a gemmologist, the refractometer indicates the refraction index of a gemstone, a telling indication of its identity. Using the example of a rubellite and ruby, the former has a refractive index of between 1.619 and 1.655 while the latter between 1.762 and 1.770. When used together with other tools of the trade, it narrows down the process of elimination quite efficiently for the gemmologist.
This weighing scale measures a gemstone’s weight in water relative to that of a body of water of equal volume. It is also able to measure specific gravity (via a formula that divides the gemstone’s weight in air by the difference between its weight in air and water), which can help identify a gemstone. Using the same comparison between a ruby and rubellite: The former has a specific gravity of between 3.97 and 4.08 while the latter has a reading of between 3 and 3.26. The majority of gemstones has a specific gravity between two and four times the equal volume of water.
A pocket-sized tool that should always be brought to gem fairs, a dichroscope is used to inspect pleochroism (colour change under polarised light) in gemstones and to differentiate between identical-looking stones (such as a ruby and a rubellite — two vastly priced gemstones). Pleochroic gems can be dichroic (showing two colours) or trichroic (three colours), depending on their crystalline systems.
Using either a prism or a diffraction grating, a spectroscope determines the optical properties of a gemstone by separating white light into spectral colours. The gemstone’s absorption spectra (revealed without the colours that are absorbed) is useful information for the gemmologist to decide from a handful of gemstones with distinctive optical characteristics.