Iridium was discovered in 1803 by the English chemist Smithson Tennant, alongside osmium, in the residues of dissolved platinum ore. He named it after Iris, the Greek goddess of the rainbow, because its salts showed so many different colours. Iridium is a hard, brittle, very dense and exceptionally corrosion-resistant member of the platinum group, the most corrosion-resistant metal known, used in spark plugs, crucibles and the tips of fine pen nibs. Able to endure great heat and almost immune to attack, iridium is used in long-lasting spark plugs, in crucibles for growing crystals, in the hard-wearing electrodes and tips of specialist instruments, and once helped define the standard kilogram in a platinum-iridium alloy.
The International Prototype Kilogram, a cylinder of ninety per cent platinum and ten per cent iridium kept at the International Bureau of Weights and Measures near Paris since 1889, served as the world's literal physical definition of mass for one hundred and thirty years, until the kilogram was finally redefined in 2019 in terms of the fixed value of the Planck constant, retiring the iridium-alloy cylinder from active duty. Iridium's near-total resistance to corrosion and stable density made it the obvious choice of alloying partner when the standard was cast in the nineteenth century.