Iridium is an extremely rare chemical element that is very hard, brittle and silvery white.
Iridium was discovered in 1803. Smithson Tennant, the primary discoverer, named the iridium for the goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes.
It is not as rare, however, for the Mogadorians who share their abundant supply with the government of the United States, who have made major scientific break-throughs. In return for the Iridium, the government aid the Mogadorians with the hunt and capture of the remaining Garde on Earth.