What causes the characteristic appearance of star sapphires, rubies, and other gemstones?
The phenomenon that causes star sapphires and rubies is called asterism. It is cause by inclusions of rutile (TiO2) crystal in the corundum (the mineral of which rubies and sapphires are varieties). Corundum grows hexagonal crystals. Thus if you were to see one, it would have a hexagonal cross section and often grows as hexagonal prisms. The rutile crystals in star corundum grows as microscopic crystal that are perpendicular to the long axis of the corundum crystal and parallel to the hexagonal crystal faces. These rutile crystals interact with light to produce the asterism. Star sapphires and rubies are the best know examples of this phenomenon, but it does occur in other minerals. It may be cause by inclusions other than rutile, but my research indicates that rutile is the most common type of inclusion that causes asterism.