Sapphire, a form of gem-quality corundum (aluminum oxide) The name sapphire usually refers to blue varieties of corundum, but the gems also occur in green, yellow, purple, and pink varieties. (Gemstones of red corundum are called rubies.) A sapphire's color is produced by traces of iron, titanium, chromium, or other metals. A star sapphire contains particles of the mineral rutile; when such a sapphire is cut and polished, light reflected by the rutile produces a star-shaped pattern.
The major producers of natural sapphires are Thailand, Sri Lanka, and Australia; India and Burma were formerly important sources. Synthetic sapphire are made commercially in several countries by a number of different processes. Sapphires are used primarily as gemstones in rings and other jewelry. Sapphire is the birthstone for September, and represents wisdom and clear thinking. Sapphires are also used as bearings in high-quality meters, compasses, and watches.
Chemical formula: Al2O3. Specific gravity: 4.00. Hardness: 9.0.