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)