Osmium is a metallic chemical element in the platinum family. This element is quite rare, and it tends to be rather expensive as a result, making commercial applications for osmium rather limited. The metal tends to be used mostly in alloys, often in very small amounts to make metal stronger and durable without driving up the cost too much. The world's major sources of this element are Turkey and Bulgaria, although it appears in small amounts in other regions as well; the element rarely appears in a pure form, but is rather combined with other metals in the form of ores.

When osmium is isolated, it is an extremely hard silvery metal with a slightly blue cast and a hexagonal crystalline structure which is very tightly packed, making it an extremely dense element. Osmium is among the heaviest of elements, along with iridium, a close neighbor on the periodic table of elements. The atomic number of osmium is 76, and the metal is identified with the symbol Os on the periodic table. Osmium is also present in an assortment of compounds, including osmium tetroxide, a toxic compound with a number of uses in the sciences.

Credit for the discovery of osmium is generally given to the English chemists Smithson Tennant and William Hyde Wollaston. The two men were working with platinum, attempting to purify the metal, in 1803 when they isolated two distinctive and unfamiliar elements, which turned out to be osmium and iridium. They officially published their findings in 1804, naming osmium for the Greek osme, “a smell,” after the distinctive odor of the compounded form of the element which they isolated.

Because osmium is so rare, its expense has made it prohibitive to work with. Therefore, few uses have been developed for the metal. It is used in an alloyed form in things like nibs for fountain pens, electrical contacts, and medical devices, since it makes metals much stronger. An osmium compound is used in fingerprint detection, and osmium tetroxide is used in chemistry and biology research labs. Osmium was also once used in light bulbs, although this usage is rare since other elements like tungsten are easier to work with, cheaper, and more effective.

This element itself is not toxic, although osmium tetroxide is extremely poisonous. Like other elements, osmium particles can cause damage to the mucus membranes because they act as an irritant. Prolonged exposure to osmium dust can damage the lungs; wearing a mask is recommended when participating in any activity with generates osmium dust.