Taranium

Taranium was the rarest mineral in the universe. It was found only on one of the outer planets of Earth's solar system (some sources specify the planet Uranus). A full emm might take fifty years to mine and process. Because of its rarity and its location, the Daleks had to recruit Mavic Chen, Guardian of the Solar System circa the year 4000 to obtain the taranium to power their ultimate weapon, the Time Destructor. The First Doctor and his companions stole the taranium Chen had gotten for the Daleks. The Daleks and Chen pursued them through time and space, finally retrieving it in ancient Egypt. (DW: The Daleks' Master Plan) Taranium was also used to construct temporal weapons on Gallifrey. (BFG: Panacea)

One account states that the Daleks used taranium to power their time machines. (DWN: Doctor Who — The Chase)

Behind the scenes
REF: The Universal Databank speculated that Kontron crystals were made of taranium.