Ruby is the red gem variety of corundum, aluminum oxide, formula Al2O3. The red comes mainly from chromium substituting for a little of the aluminum. Pink corundum is the same mineral species; the name ruby is reserved for the stronger red gem material.
Corundum is trigonal, hardness 9, and has no cleavage, though it may part along planes. Crystals are often barrel-shaped hexagonal prisms. Ruby is denser than most red look-alikes such as garnet or spinel, and it is harder than both.
This page is only for ruby. Sapphire is the other gem variety of the same mineral species and has its own page.