Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide (α-Al2O3). Whilst typically associated with the color blue, natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color which sapphire cannot be is red - as red colored corundum is called ruby, another corundum variety.
Trace amounts of elements such as iron, titanium, chromium, copper, or magnesium present during formation are responsible for the color of a sapphire. Chromium impurities in corundum yield a pink hue.
Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules.
Because of the remarkable hardness of sapphires – 9 on the Mohs scale (the third hardest mineral, after diamond at 10 and moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such infrared optical components; high-durability windows; wristwatch crystals and movement bearings; and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (especially integrated circuits and GaN-based LEDs).
Sapphire is the birthstone for September and the gem of the 5th and 45th anniversaries.