Aluminium

Aluminium was a metallic element. It could be found on such planets as Earth (TV: Dream-Eaters) and Megerra. (PROSE: Mean Streets)

The psychic energy given off by the Bodach could be blocked by aluminium, especially in the form of hats. (TV: Dream-Eaters)

Erato wove a shell of aluminium around a neutron star that was set to destroy Chloris, allowing the Fourth Doctor to use his TARDIS' tractor beam to pull it off course. (TV: The Creature from the Pit)