Iridium is a metallic element with the atomic number 77 and the symbol Ir. On the periodic table of elements, it is found in Group 9, between platinum and osmium. It is referred to as a “transition metal” and also as a “platinum metal,” along with ruthenium, rhodium, palladium, osmium, and platinum itself.

Iridium is a metallic element that is both hard and brittle. Described as whitish-yellow or silvery in color, it just a little less dense than osmium, which is the densest element—22.560 gm/cm³ compared to 22.6 gm/cm³ .

Iridium was discovered by Smithson Tennant, an English chemist, in London, England in 1803, and it’s name comes from the Latin for iris, the goddess of the rainbow on account of its colored salts. Tennant was also he discoverer of osmium.

It is found in gravel deposits, and believed to occur only in alloys with other noble metals, i.e., not in an uncombined state. Often it is found alloyed with osmium in compounds called osmiridium and iridiosmium, the origin of the names being obvious. It is, however, extremely scarce, occurring in only .001 ppm in the Earth’s crust.

Iridium has a number of uses. It is used to harden platinum, combined with osmium in creating gold-tip pens, and to make highly specialized crucibles. It iridium can be involved in cancer irradiation, as well as the making of hypodermic syringes and surgical pins. Other uses include spark plugs for helicopters, pivot bearings, and extrusion dies.

One very special use of iridium is its role in the weights and measures standard for the kilogram, which is made of an alloy of iridium (10%) and platinum (90%). On account of exceptionally high amounts of iridium in rocks dating to between the Cretaceous period and the Tertiary period, t is speculated by some that iridium was a component of an asteroid that struck the Earth and led to the extinction of the dinosaurs.