Osmium was discovered in 1803 by the English chemist Smithson Tennant, in the dark residue left when crude platinum was dissolved in acid, the same residue that yielded iridium. He named it from the Greek osme, a smell, after the sharp, acrid odour of its volatile oxide. Osmium is a hard, brittle, bluish-white member of the platinum group and the densest of all naturally occurring elements, used in tiny quantities to harden alloys for fountain-pen nibs and instrument pivots. The densest of all naturally occurring elements, osmium is hard and brittle and used only in small amounts to harden platinum alloys, in the tips of fine pen nibs, instrument pivots and electrical contacts, while its volatile oxide is a valued stain in microscopy.
Osmium tetroxide is also a standard fixative and stain in electron microscopy, since it binds preferentially to fats and lipids in biological tissue, rendering cell membranes visible at the extreme magnifications the technique requires, a use that makes this pungent, toxic compound as familiar in biology laboratories as the metal itself is scarce in industry. Its notorious volatility and toxicity mean elemental osmium is handled far more cautiously than its platinum-group relatives, almost always alloyed rather than used in pure metallic form.