Taranium

Taranium was the rarest mineral in the universe.

Properties
Taranium was very unstable and capable of storing vast amounts of energy. It was used in time travel and could send whole areas back in time. It was incredibly rare and would require processing over a billion tonnes of ore to collect a milligram. One of the few planets it was found on was Uranus. (PROSE: The Wheel of Ice) It could also be found in Pazithi Gallifreya. (PROSE: Cat's Cradle: Time's Crucible)

Usage
In the 21st century, the Second Doctor planned to help Arkive send a message back in time by having MMAC collect taranium from Uranus. (PROSE: The Wheel of Ice)

The Daleks used taranium to power their time machines. (PROSE: Doctor Who — The Chase) 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. (TV: The Daleks' Master Plan)

Taranium was also used to construct temporal weapons on Gallifrey. (AUDIO: Panacea)

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