One of the polymorphs, with rutile and brookite, of titanium oxide, TiO2, anatase is brown or blue in colour and crystallizes in the tetragonal sys-tem, usually as bipyramids. The form often resembles the octahedron.Anatase has a hardness of 5.5–6, an SG between 3.82 and 3.95, and RI 2.493 for the extraordinary ray and 2.554 for the ordinary ray. The nega-tive birefringence is 0.061. Anatase resultsfrom the alteration of othertitanium-bearing minerals and is also found in hydrothermal veins.Localities for anatase include Isère, France; gemmy material from Binn,Valais, Switzerland, the Urals and Minas Gerais in Brazil. Other localitiesare Massachusetts, North Carolina and Colorado in the USA, where bluecrystals are found.