Voolium

Voolium was a mineral abundant on the planet Calufrax, which, according to the Fourth Doctor, was the only place it was known to occur naturally.

The vibrations of refined voolium and madranite one-five crystals could be used to power a psychic interference transmitter, and the presence of these two minerals brought Calufrax to the attention of the crew of the pirate planet Zanak, who needed it to neutralise the psychic powers of the Mentiads. (TV: The Pirate Planet)

Behind the scenes

 * The Captain seems to call voolium "vasilium" at the start of the first episode of The Pirate Planet, which is probably either a mistake in the script or by the actor.