Iridium's atomic number is 77 and its symbol is Ir. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is considered to be the second-densest naturally occurring metal (after osmium). It is the most corrosion-resistant metal, even at temperatures as high as 2000 °C.

Named iridium after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is found in meteorites in much higher abundance than in the Earth's crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs.

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