Pumice

From Tardis Wiki, the free Doctor Who reference
Pumice
Containers of clay and pumice. (TV: The Doctor Dances)

Containers of clay and pumice were present in a storeroom within which the Ninth Doctor and Rose Tyler were trapped after escaping the patients infected (TV: The Doctor Dances) with the "Empty Child Syndrome". (PROSE: Amarillo Hoax, Hitler's Last Secret?)