In gemmology, an asterism is an optical phenomenon isplayed by some rubies, sapphires, and other gems (i.e. star garnet, star diopside, star spinel, etc) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone. Star sapphires and rubies get their asterism from the titanium dioxide impurities (rutile) present in them.