Demantoid

The name carries a promise. A green garnet that throws so much fire made its early observers reach for the one comparison everyone understood — diamond. Demantoid is the green gem variety of andradite, a calcium-iron member of the garnet family.

Garnets had been worn since antiquity, but this brilliant green kind was not found until 1851, in the western central Ural Mountains of Russia. The first stones came from an alluvial deposit — gravel washed and concentrated by water — along the Bobrovka River. The site lies about 110 km north-northwest of Ekaterinburg, near the village of Elizavetinskoye. A second source was later worked to the south of the city, on the Chusovaya and Chrisolitka rivers.

The Finnish mineralogist Nils Gustaf Nordenskiøld gave the gem its name a few years after the find. He took it from the old German word Demant, meaning diamond, in tribute to the stone's diamond-like brilliance. The grass-green to emerald-green colour comes from a trace of chromium sitting in the iron framework of the crystal.

The timing was good. From the discovery until about 1919, demantoid was fashionable in Russia, and the celebrated court jeweller Peter Carl Fabergé set the green stones into his work. After the Russian Revolution, in the austerity of Communist Russia, the gem fell out of fashion.

The horsetail signature

Russian demantoid carries a clue to its origin inside the stone itself. Many crystals enclose fine fibres of chrysotile, a form of asbestos, that radiate outward from a tiny central crystal of chromite. The golden threads curve as they spread and look like the tail of a horse, which is why cutters call them horsetail inclusions. Gemmologists treat the horsetail as diagnostic: it confirms a natural demantoid, because the feature is not found in any other green gemstone.

For most of the 20th century the Ural mines were the only meaningful source. That changed in 1996, when a major find of demantoid and andradite came to light in Namibia, at the deposit now called the Green Dragon mine. Around 2009 another significant discovery followed in Madagascar.

Demantoid has no industrial use. It is a gemstone, cut and set for its beauty alone, and prized by collectors of fine coloured stones.

Its appeal rests on fire — the way a faceted stone splits white light into flashes of rainbow colour. Demantoid disperses light unusually strongly, with a dispersion value of 0.057, higher than diamond's, so the effect is easy to see. A high refractive index, between 1.80 and 1.89, adds to the brilliance by bending and returning more of the light that enters the stone.

Fine demantoid is scarce, which keeps it firmly in the jewellery and collector market rather than any bulk trade. Finished stones are usually under 1 carat, or 200 milligrams; cut stones above 2 carats are rare, and those above 3 carats very rare. Among Russian material, the curling horsetail inclusions that mark a natural stone are sought after rather than hidden, and can raise its value when clearly visible.