Iridium is a metallic element much like platinum. It was identified in 1802. With the exception of osmium, also a newly discovered metal, it is the heaviest substance known. It is very difficult to melt, requiring a temperature of at least 3600 °F. It is insoluble in acid and does not rust. It is used frequently for the tips of gold and stylographic pens. Also in the construction of electric instruments, as the resistance coil. Small quantities are obtained in California, Borneo, Brazil, and in the Ural Mountains. It forms three oxides. The solutions made from the salts of the metal take on various tints; hence the name, from Iris, the rainbow.