A very hard, brittle, silvery-white metallic chemical element with the symbol Ir and atomic number 77, highly resistant to corrosion and extremely rare in Earth's crust.
Named from Greek 'iris' (genitive 'iridos') meaning 'rainbow' due to the varied colors of its compounds. English chemist Smithson Tennant discovered and named the element in 1803, the same year he discovered osmium. The rainbow reference reflects the striking array of colors produced by different iridium compounds.
Iridium is so rare on Earth that scientists use it as evidence for asteroid impacts - the famous 'iridium layer' in rock formations marks the asteroid that killed the dinosaurs 66 million years ago! Despite being named for rainbows, pure iridium metal is actually quite dull and silvery.
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