Sphene

Sphene is the silicate and titanate of calcium, and its chemical formula is CaTiSi05. One of the less-known and rarer gem stones, it occurs in yellow, green, and brown shades. Despite its hardness of only 5,5  its truly exciting fire, which exceeds that of the diamond, makes it a valuable and much-desired gem. Sphene rarely occurs in gem-quality crystals, and those that have been found come chiefly from Austria, Switzerland, Mexico, and Brazil.