Short answers, pulled from the story.
The British chemist Smithson Tennant discovered iridium in 1803 in the acid-insoluble residues of platinum ores. He documented the discovery in a letter to the Royal Society on the 21st of June 1804, identifying both iridium and osmium in the black residue.
Iridium takes its name from Iris, the Greek winged goddess of the rainbow and messenger of the Olympian gods. Smithson Tennant chose the name because many of the salts he obtained from the metal were strongly colored.
Iridium has a density of 22.56 grams per cubic centimeter, making it the second-densest naturally occurring metal after osmium at 22.59. It is one of the nine least abundant stable elements in Earth's crust, averaging 0.001 parts per million in crustal rock, with about 15,000 pounds produced in 2023.
A thin layer of iridium-rich clay marks the Cretaceous-Paleogene boundary 66 million years ago. In 1980 a team led by Luis Alvarez proposed this iridium came from an asteroid or comet impact, the Alvarez hypothesis now widely accepted to explain the extinction of the non-avian dinosaurs, later linked to the Chicxulub crater beneath the Yucatán Peninsula.
Iridium is used in high-performance spark plugs, crucibles for growing oxide single-crystals such as sapphires, electrodes for producing chlorine, and catalysts including the Cativa process for making acetic acid. It is also a component of some OLEDs and was used in the International Prototype Meter and kilogram as a 90 percent platinum, 10 percent iridium alloy.
Iridium-192, with a half-life of 73.82 days, is used in brachytherapy to treat cancer and in industrial radiography for non-destructive testing of welds in steel for the oil and gas industries. It is normally produced by neutron activation of iridium-191 in natural-abundance iridium metal.
Iridium is the most corrosion-resistant metal known and is not attacked by acids, including aqua regia, even at temperatures as high as 2000 degrees Celsius. It can only be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate, or reacted with halogens, sulfur, and cyanide salts under specific conditions.