Taranium

Taranium was the rarest mineral in the universe, one account claiming that it could only be found on one of the "dead planets" of the Solar System, later specified as Uranus. (TV: The Daleks' Master Plan) However, the mineral could be found elsewhere, including on Gallifrey's moon (PROSE: The Scrolls of Rassilon) of Pazithi Gallifreya. (PROSE: Cat's Cradle: Time's Crucible)

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. (PROSE: The Wheel of Ice) It took the Daleks two decades to acquire a gram of taranium, which could power their timeship for centuries. (PROSE: The Chase) and it took Mavic Chen 50 years to acquire an emm of the mineral. (TV: The Daleks' Master Plan)

Usage
Rassilon's Time Scoop used taranium as a fuel source. (PROSE: The Scrolls of Rassilon)

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: The Chase, Mission to the Unknown / The Mutation of Time) The First Doctor was disdainful of the Dalek time machine on Mechanus when he had a chance to examine it when the Daleks and the Mechonoids destroyed each other. He pointed out to Vicki that it was powered by taranium. (PROSE: 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 acquired for the Daleks. The Daleks and Chen pursued them through time and space, finally retrieving it in ancient Egypt. It burnt itself out when the Time Destructor destroyed Kembel. (TV: The Daleks' Master Plan)

During the chase, the Doctor encountered a time traveller named Robert who had aged to about 30 years old, but whose mind was still that of an 8-year old. Upon grasping the taranium, it changed the man's body back to 8 years old, matching his mind. (PROSE: The Little Drummer Boy)

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

When the Eighth Doctor hooked up his TARDIS to the engines of the Dalek Time Squad's time machine, he asked for the Scientist to help monitor the instability that the taranium would experience when they travelled back to the Dark Times. (AUDIO: The Enemy of My Enemy)

Behind the scenes

 * The reference book The Universal Databank speculated that Kontron crystals were made of taranium.
 * In The Little Drummer Boy, Robert is de-aged by contact with the taranium core and is thus taken back in time by the Doctor to replace his ill brother Christopher, suggesting that exposure to the taranium rendered Robert permanently immune to the Blinovitch Limitation Effect.