Demantoi is the green gemstone variety of the mineral andradite, 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.