The following information is from the website:- http://en.wikipedia.org/wiki/Demantoid
Demantoid is the green gemstone variety of the mineralandradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet. It is the most expensive and rare of garnet gemstones, with fine specimens commanding prices of thousands of dollars per carat (thousands per gram). The chemical formula is Ca3Fe2(SiO4)3 with chromium substitution as the cause of the demantoid green color. Ferric iron is the cause of the yellow in the stone.
It has the misnomersolivine,[2] and Uralian emerald.
In approximately 2003, reports began to circulate in the trade[3] that some Russian Demantoid garnets were being routinely subjected to heat treatment in order to enhance their colour. Such treatment is believed to be performed at relatively low temperatures and is not thought to be detectable by gemmological testing.
The hardness (Mohs scale) : 6.5
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