Zeiton

Zeiton was an incredibly rare element from the planet Varos. The Thirteenth Doctor referred to specific pieces of Zeiton as Time Crystals. (GAME: The Edge of Reality) It was a class of mineral and fuel which was used by TARDISes, types of which included Zeiton-4 and Zeiton-7. The Tenth Doctor fired up the Zeiton crystals as one of the steps to keep his TARDIS and his fifth incarnation's TARDIS from blowing a hole in the space-time continuum. (TV: Time Crash)