Osmiridium, also known as iridosmium or iridosmine, is a natural alloy of osmium and iridium, with traces of other platinum group metals. Osmiridium has been defined as containing a higher proportion of iridium, while iridosmine contains more osmium. However, as the content of the natural Os-Ir alloys varies considerably, the constituent percentages of specimens often reflects the reverse situation of osmiridium describing specimens containing a higher proportion of osmium and iridosmine containing more iridium. In 1963, Max Hutchinson Hey proposed using iridosmine for hexagonal specimens with 80%>Os>32%, osmiridium for cubic specimens with Os<32% and native osmium for specimens Os>80%. More recently, the Nomenclature Subcommittee of the Commission of New Minerals and Mineral Names, International Mineralogical Association has declared the mineral names osmiridium and iridosmine invalid and to be replaced by iridium and osmium, respectively. Other named naturally occurring alloys of platinum metals have included: iridrhodruthenium, platiniridium, ruthenosmiridium, and rutheniridosmine. The properties of these alloys generally fall between those of the members, but hardness is greater than the individual constituents.
Osmiridium is very rare, but it can be found in mines of other platinum group metals. One very productive mine was operated at Adamsfield near Tyenna in Tasmania during the Second World War with the ore shipped out by railway from Maydena. The site of the mine is now totally reclaimed by dense natural bush. It was once one of the world's major producers of this rare metal, and the osmiridium was mostly found in shallow alluvial workings.
It can be isolated by adding a piece to aqua regia, which has the ability to dissolve gold and platinum but not osmiridium. It occurs naturally as small, extremely hard, flat metallic grains with hexagonal crystal structure.