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 the Mt. Vesuvius volcano.
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 is from the Greek and means mixed form, a reference to its crystals showing a mixture of other mineral forms.
Idocrase is not only rare, but transparent specimens that can be faceted count as very rare indeed. 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.
Gemologically, idocrase is a 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 spinel and garnet. Idocrase is slightly softer than quartz, with a rating of 6.5 on the Mohs scale.
Idocrase or vesuvianite is found in a number of locations in the world. They include Quebec in Canada, Mt. Vesuvius in Italy, the Ural Mountains in Russia, Switzerland, Kenya and Tanzania. But 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 California Jade.
- First Published: March-25-2009
- Last Updated: March-04-2011
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