: a silver-white hard brittle very heavy metallic element of the platinum group used especially in hardening platinum for alloys (as for surgical instruments)—symbol Ir; see element table
Metallic chemical element, one of the transition elements, chemical symbol Ir, atomic number 77. A very rare, precious, silvery white, hard, brittle metal that resists most acids, it is one of the densest substances known on Earth. It probably does not occur uncombined in nature but is found in natural alloys with other noble (i.e., chemically inactive or inert) metals. The pure metal is too hard to work with to have any significant uses; alloys with platinum are used in jewelry, pen points, surgical pins and pivots, electrical contacts and sparking points, and extrusion dies. The international prototype kilogram, the primary standard (seeweights and measures) for weight, is made of an alloy comprising 90% platinum and 10% iridium. The discovery of abnormally high amounts of iridium in rocks dating to the boundary between the Cretaceous and Paleogene periods led to a much-debated hypothesis that an iridium-containing asteroid striking Earth led to a catastrophic chain of events including the extinction of dinosaurs and many other forms of life.