Thaesium
From Tardis Wiki, the free Doctor Who reference
Thaesium was a radioactive crystalline mineral used by Solonians to cause the "mutations" that were part of their life cycle. It was also used as fuel by humans during the time of the Earth Empire. Solos was rich in thaesium, and the planet was mined by the Empire until the deposits were nearly exhausted. (TV: The Mutants)
