Galdrium

Galdrium was a mineral.

Galdrium was one of the minerals mined by the planet Zanak. (TV: The Pirate Planet)

Galdrium was from Tridentio III. It looked like gold but could shine in the dark if shaken. The Eighth Doctor had a dozen or two pieces of it. (AUDIO: The Witch from the Well)

There were also deposits of galdrium on Cawdor. (AUDIO: The Acheron Pulse)