Validium

Deadly living metal which was allegedly stolen by the Doctor before he left Gallifrey. Validium was to be the Time Lords's ultimate defense. After it was used in a series of unknown circumstances, which again involved the Doctor, validium landed on Earth in Windsor in 1638 AD. There, it was shaped into a statue dubbed Silver Nemesis by Lady Peinforte. The Nemesis revealed many secrets about the Time Lords and the Doctor to her. The Seventh Doctor eventually used the Silver Nemesis to destroy the Cybermen's space fleet, then ordered it to reshape itself.