Apatite 

is one of a few minerals produced and used by biological micro-environmental systems, apatite is the defining mineral for 5 on the Mohs scale of hardness. Apatite is infrequently used as a gemstone, transparent stones of clean color have been faceted, and chatoyant specimens have been cabochon-cut, chatoyant stones are known as cat's-eye apatite.

Source: Wikiepida