iridosmine, also called Osmiridium, mineral consisting of an alloy of iridium and a smaller proportion of osmium. It occurs in gold-bearing conglomerates, as at the Witwatersrand in South Africa, and in gold sands, as in California and Oregon, U.S. Because of their hardness and resistance to corrosion, both natural and synthetic iridosmine are used for tips of pen nibs, surgical needles, and sparking points in engines. Similar alloys composed of more osmium than iridium are called siserskite. Both iridosmine and siserskite crystallize in the hexagonal system. For detailed properties, see native element (table).