Validium

Validium was a Time Lord superweapon in the form of a living metal created as the ultimate defence for Gallifrey by both Rassilon and Omega and had great destructive potential and could also think for itself and was capable of speech as well. When destroyed, it could even reform itself. (TV: Silver Nemesis)

History
When Rassilon and Omega found that the validium's sentience made it too difficult to control, they locked it away in an impenetrable vault where it was casually stolen. (PROSE: A Brief History of Time Lords) When Rassilon was building the Type 1 TARDISes, validium was used in their fluid links. (COMIC: The Lost Dimension)

It should never have left Gallifrey, but some of it did. A piece of validium fell to Earth and was found by the Lady Peinforte, who moulded it into a statue of herself holding a bow and arrow and called it Nemesis. The Doctor then launched the Nemesis into space on the rocket-sled, though the bow and arrow were kept on Earth. Its orbit had brought it near Earth every 25 years, each time causing disasters by remote influence. In his seventh incarnation, the Doctor arrived on Earth as it approached again in 1988.

Following the Doctor's instructions, the Nemesis was launched to rendevous with the Cyberman fleet in Earth orbit and destroyed the fleet. It reformed in space. (TV: Silver Nemesis)

The Cold was made of validium. (PROSE: Interference - Book One / Book Two)

After the Lethean Campaign, (PROSE: The Book of the War) a piece of validium was left behind on Lethe, where it was made into a staff. (PROSE: The Parliament of Rats)

Martyn Townsend told Ciara that he wanted to know what properties validium offered his organisation. (PROSE: Business Unusual)

Alternative timelines
On an alternative Gallifrey, Rassilon placed the Eye of Harmony and a prototype TARDIS in a dome of validium which used gravity waves to keep the Eye from working, thus preventing the Krilig from escaping. (AUDIO: Forever)