Which compound is the main mineral in the gemstones ruby and sapphire?

The main mineral in ruby and sapphire is aluminium oxide.

Aluminium oxide, also called alumina, has the formula Al₂O₃ and occurs naturally as the mineral corundum. Pure corundum is colorless, but trace impurities create the gemstone colors. Chromium gives ruby its characteristic red color, while iron and titanium commonly contribute to sapphire’s blue color.

Ruby and sapphire are therefore varieties of the same mineral rather than completely different compounds. In gem classification, red corundum is called ruby; corundum of other gem colors is generally called sapphire. The boundary is based mainly on color, not on a different basic chemical formula.

Alumina is also important outside gemology. It is a very hard ceramic material and an electrical insulator, and it is used in abrasives, refractories, laboratory equipment, and aluminium production. Its hardness helps explain corundum’s place near the top of the Mohs scale.