Gemologically, Idocrase is a complex mix of elements including oxygen, hydrogen and fluoride. It belongs to the silicate group of minerals and is defined more specifically as a calcium aluminum silicate. Its specific gravity ranges from 3.32 to 3.47, also in a range similar to peridot. With a hardness of 6.5, Idocrase is slightly softer than quartz. Although Idocrase crystalizes in a tetragonal structure massive forms are not uncommon and African material may be difficult to distinguish from some forms of grossularite. The vitreous to resinous luster of Idocrase is another of its distinguishing features.
Idocrase is found at a number of locations throughout the world and may be related to limestone deposits, contact metamorphism, or volcanic activity. Some of the most notable gem quality deposits come from Quebec, Canada, and Mt. Vesuvius in Italy. Other sources include the Ural Mountains of Russia as well as China, Norway, Pakistan, Sweden, Switzerland, Kenya, Tanzania and in the United States; California, Arkansas, Maine, Vermont and New Hampshire.
More recently, finds of Kenyan and Tanzanian Idocrase have brought some fairly transparent specimens to the market but not in significant quantities. The Jeffrey Mine in Quebec, produces a fine facet grade yellow-green material as well as a chromium-rich emerald-green variety and an extremely rare violet to lilac colored variety. A massive compact form resembling jade is referred to as “californite” or “California jade”. Chromium-rich idocrase is known as chrome vesuvianite, and a rare blue variety is known as cyprine. “Mangan vesuvianite” is a reddish to purple variety of manganese-rich idocrase.
In the world of metaphysical gems, idocrase is more often referred to as Vesuvianite. It is an astrological gemstone associated with the signs of Sagittarius and Capricorn. It is believed to help clear negative energy and thoughts, and to alleviate anger, depression and fear.
Idocrase is always popular among collectors but less known in the mainstream market. It is not known to be treated or enhanced in any way and is among the gemstones which remain completely untreated from the mine to the market.