Idocrase is a rare gemstone, usually found in shades of green but sometimes in yellow-brown or pale-blue. Idocrase belongs to the silicate group of minerals. While the name idocrase is used for rare gemstone-quality specimens, the mineral is usually known by the name vesuvianite, since the first samples were found on Mount Vesuvius.
The mineral was first identified and named by the famous German gemologist Abraham Gottlob Werner in 1795. Werner was also the first to identify chrysoberyl, and he was the mentor of Friedrich Mohs, inventor of the Mohs scale of mineral hardness. The name idocrase comes from Greek words meaning 'mixed form', in reference to its crystals showing a mixture of other mineral forms.
Idocrase is not only an uncommon material, but transparent specimens that can be faceted are extremely rare. Most gem-quality idocrase is opaque with an appearance similar to jade. The opaque specimens have a greasy or resinous luster, while the rare transparent form has a vitreous luster.
Idocrase is composed of calcium magnesium iron aluminum silicate hydroxide. It has a specific gravity of 3.32 to 3.47, about the same as tanzanite. Idocrase has a refractive index of 1.700 to 1.723, slightly lower than that of spinel and garnet. With a rating of 6.5 on the Mohs hardness scale, idocrase is slightly softer than quartz.
Idocrase or 'vesuvianite' is found in a number of locations in the world. They include Quebec in Canada, Mount Vesuvius in Italy, the Ural Mountains of Russia, Switzerland, Kenya and Tanzania. However, there are very few sources of gem-quality crystals. Recent finds in Kenya and Tanzania have brought some very fine stones to the market, albeit in limited quantity. One source of cabochon grade material is California, USA. It has been marketed as Californite or rather misleadingly, as 'California jade'.
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