Iridium

Iridium is named after Iris, the Greek goddess of the rainbow, because its salts produce a wide variety of vibrant colors. Discovered in 1803 by Smithson Tennant (at the same time as osmium), iridium is one of the rarest elements in Earth's crust — but it is much more common in meteorites, which is a major clue to one of science's greatest mysteries.

A thin layer of iridium-rich clay found worldwide marks the boundary between the Cretaceous and Paleogene periods (66 million years ago) — this 'iridium anomaly' is key evidence that an asteroid impact wiped out the dinosaurs. Iridium is extremely corrosion-resistant and was used to make the standard kilogram prototype in Paris. Today it is used in spark plugs, crucibles for growing crystals, and in some cancer treatments.

Iridium's symbol 'Ir' is a handy two-letter element in Spellement. While 'ir' at the start of words is less common than some pairs, it appears in words like 'iron' and 'iris.' Partial emphasis provides access to 'i' or 'r' individually, both high-frequency English letters.