Taranium

Taranium is the rarest mineral in the universe. It is found only on one of the outer planets of Earth's solar system, - some sources specify the planet Uranus, - and a full emm may 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 necessary to power their ultimate weapon, the Time Destructor. When the Doctor and his companions succeeded in stealing the taranium Chen had obtained 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 in use on Gallifrey. (BFG: Panacea)

Possibly apocryphal information
One account states that the Daleks used taranium to powered their space-time vessels, the DARDISes. (DWN: Doctor Who — The Chase)