Iridium

Iridium is rare in the Earth's crust because it is siderophile — nearly all of it sank into the core during differentiation — but it is comparatively common in chondritic meteorites. That contrast is what let Luis and Walter Alvarez, in 1980, interpret a thin iridium-rich clay layer found at the Cretaceous-Palaeogene boundary all over the world as the fallout of a massive impact; the Chicxulub crater was identified a decade later. Iridium is the most corrosion-resistant metal known, unattacked by any acid including aqua regia, and its extreme hardness plus a 2466 °C melting point makes it the crucible material for growing sapphire and oxide laser crystals. Iridium-192 is the workhorse gamma source for industrial radiography of pipeline welds.