Paxltt

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