Paxltt

 was a mineral unique to the unnamed planet inhabited by the Tryods and an un-named humanoid species. It was found very close to the surface of the part of the world on which the Tryods lived. The humanoid species had wanted to massacre all the Tryods so they could get access to this mineral. The Second Doctor identified it as a mineral unique to the planet after the humanoid, Asiries, gave him a lump of it, promising never to throw it at the Tryods again. (PROSE: The Word of Asiries)