Next to diamond, corundum is the hardest natural mineral found on earth.1 It is an aluminum oxide (Al2O3), and like beryl takes its coloration from various impurities. The name probably comes from the Sanskrit word for ruby. The most common form of corundum is emery, which was used by medieval jewelers to polish other gemstones. Red corundum is called ruby, while blue corundum has been known as sapphire since medieval times. Corundum in other colors was formerly known as "Oriental", such as "Oriental emerald" (green corundum), "Oriental topaz", "Oriental amethyst", etc. Since the beginning of the 20th century however, all corundum other than ruby has been known as sapphire. Corundum ranks 9 on the Mohs scale of hardness, and has no cleavage planes. It is extremely durable making it suitable for any jewelry application, though due to its extreme hardness it is seldom used in beads.
1. Silicon carbide (SiC) is harder than corundum (about 9¼). It is believed to be associated with meteorites, and deposits have been found in Meteor Crater, AZ. These deposits are too small to be commercially exploitable, however. In man-made form it is known as moissanite.
June Culp Zeitner, Gem & Lapidary Materials, Geoscience Press, 1996.