What causes the Star in Star Sapphires?
Sapphire, like all other forms of corundum, is composed of aluminum oxide (Al2O3) and rutile (TiO2). The rutile within the sapphire and can appear as a star pattern. Formally called an asterism, the Star occurs when fibers of rutile intersect in either a six-ray or a twelve-ray pattern. The star is only visible when the included sapphire is cut into a cabochon. Heat treating the sapphire clears most of the rutile from the stone and the star will disappear.