Iridium

Iridium was a metallic element.

Matthew O'Connell looked for iridium and gold while mining. (PROSE: Dreamstone Moon)

An iridium alloy was used in the construction of the De-mat Gun, which was built by Rodan and K9 Mark I by order of the Fourth Doctor and powered by the Great Key of Rassilon. (TV: The Invasion of Time)