Corundum - Al₂O₃

Habit: Commonly prismatic, often barrel-shaped crystals, also tabular or rhombohedral, massive or granular. Can be any color, but most notable is several shades of blue (sapphire), pigeon blood-red (ruby).Transparent to translucent, Adamantine to vitreous Luster.

Environment: Characteristically found in silica-deficient rocks such as syenites and their associated Pegmatites. Also in mica schist, gneiss, and some marbles in metamorphic terranes.

Etymology: Probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam.

The principle use of corundum is as an abrasive, normally called alumina. It is used as a gem, and synthetic crystals are used in bearings for watches and fine instruments, and for lasers.