Metallic chemical element , chemical symbol Al, atomic number 13.
A lightweight, silvery white metal , it is so reactive chemically that it always occurs in compounds. It is the most abundant metallic element in Earth's crust, chiefly in bauxite (its principal ore ), feldspar s, mica s, laterite . It also occurs in gemstones, such as topaz , garnet , and chrysoberyl; emery , corundum , ruby , and sapphire are crystalline aluminum oxide . Aluminum was first isolated in 1825, became commercially available in the late 19th century, and is now the most widely used metal after iron . Its surface oxidizes at once to a hard, tough film, deterring further corrosion. Uses include building and construction, corrosion-resistant chemical equipment, auto and aircraft parts, power transmission lines, photoengraving plates, cookware and other consumer goods, and tubes for ointments and pastes. Important compounds include alum s; alumina (aluminum oxide), useful as corundum and as a carrier for many catalysts; aluminum chloride, a widely used catalyst for organic syntheses; and aluminum hydroxide, used to waterproof fabrics.