Indium, a chemical element, symbol Ir; for physical constants see Periodic Table. Iridium was discovered by Smithson Tennant in 1803. It occurs in the presence of platinum and other metals in alluvial deposits. It is obtained as a byproduct of nickel smelting. Iridium is a silvery-white, hard, brittle, unreactive, metal. It is the most corrosion-resistant metal known. The metal is used in high-melting, hard, and unreactive alloys. Iridium and its compounds are used in crucibles, pen nibs, electrical contacts, and organic catalysts.