Iridium is a very hard, brittle, silvery-white transition metal of the platinum group, second-densest element.
Core Principles & Analysis
Smithson Tennant discovered iridium in 1803 alongside osmium, naming it after Iris, goddess of the rainbow, for its colorful salts.
It is the most corrosion-resistant metal known, and the worldwide iridium layer at the Cretaceous-Paleogene boundary underpinned the asteroid-impact extinction hypothesis.
Iridium tips long-life spark plugs, lines crucibles for growing single crystals, and alloyed with platinum formed the international prototype kilogram.