Sapphire

Sapphire, precious gemstone that occurs as a transparent, blue variety of the mineral corundum. The ruby is a red variety of gem-quality corundum. Although the term sapphire is often applied to gem-quality varieties of corundum of all other colors, the true sapphire is deep blue, the best tint being a cornflower blue called Kashmīr blue. Colorless, precious corundum is called white sapphire; yellow corundum is called yellow or golden sapphire, or Oriental topaz; and pale pink stones are called pink sapphire. The various colors in corundum are caused by small amounts of impurities, such as chromium, iron, or titanium, in the aluminum oxide (Al₂O₃) of which the mineral is essentially composed. Some sapphires exhibit asterism when cut cabochon, that is, in convex form and not faceted, and are highly valued star sapphires. Sapphires are used, as are rubies, as hard bearings in watches and scientific apparatus. Synthetic sapphires are produced in a manner similar to that used for synthetic rubies.