(Chemical Symbol: Os) A bluish-white, hard, crystalline metallic element belonging to the platinum family of elements. Discovered in 1803 by Tennant, it is used as a hardening alloy in platinum. Osmium is used for fine machine bearings, for pen points and instrument pivots. With iridium, it forms an alloy, osmiridium, which is used for making filaments in incandescent lamps. Atomic number: 76; atomic weight: 190.; melting point: 3000°C (5432°F); specific gravity: 22.48 at 20°C (68°F).