Corundum

Corundum, mineral consisting of alumina (aluminum oxide, Al₂O₃) and second in hardness only to diamond. It forms precious stones, such as rubies, sapphires, topaz, amethyst, and emerald; the “jewels” in watches and instrument bearings are often made of corundum. Because of its hardness, corundum is widely used in emery and other abrasives. Corundum is found in many countries, often in association with quartz.