Osmium

Osmium is a hard, brittle, bluish-white metal in the platinum family, and the densest element found in nature: about 22.6 grams per cubic centimeter, roughly twice as heavy as lead. Smithson Tennant discovered it in 1803 in the black residue left after dissolving platinum ore in acid. He named it after the Greek osme, smell, because its oxide has a sharp odor.

Osmium is extremely rare and is mostly obtained as a by-product of nickel and platinum mining. Its tough alloys, often with iridium, were used for fountain-pen tips and electrical contacts. Osmium tetroxide is poisonous but useful: biologists use it to stain fats so cells show up clearly under the electron microscope.