British chemist Smithson Tennant identified iridium in 1803 and published his findings in a letter to the Royal Society on the 21st of June 1804. Earlier attempts by French chemists Victor Collet-Descotils and Antoine François de Fourcroy had failed to isolate enough material for study.
Iridium possesses a density of approximately twenty-two grams per cubic centimeter according to X-ray crystallography data. It remains the second-densest naturally occurring metal after osmium.
A thin layer of clay marking the Cretaceous-Paleogene boundary contains unusually high concentrations of iridium dating back 66 million years. Luis Alvarez proposed a theory in 1980 suggesting an asteroid impact caused the extinction of non-avian dinosaurs based on these elevated levels.
Major reserves exist within the Bushveld igneous complex in South Africa near the Vredefort impact structure. Significant deposits also appear in Russia near Norilsk and Canada within the Sudbury Basin.
An alloy containing ninety percent platinum and ten percent iridium constructed the International Prototype Meter bar in 1889. That metal bar remained the standard for length until 1960 when atomic spectra took over, while a similar alloy defined the kilogram mass standard until May 2019.