The Stone of Scone was a psychic lifeform native to the planet Micah, and a stone block which was used for centuries in the coronation of the monarchs of Scotland, England, and later Britain. (PROSE: Set in Stone)
The Ninth Doctor witnessed the coronations of Macbeth and Gruath on the stone, before remarking to Gruath that if the people knew what the stone really was, they wouldn't go near it. (AUDIO: The Curse of Lady Macbeth)
Indeed, on 25 December 1950, the First Doctor, Ian Chesterton and Barbara Wright took the stone from Westminster Abbey to the Doctor's TARDIS in Kilmartin, Scotland. The Doctor had seen a photograph of the stone and recognised it as a psychic life form native to Micah, and believed that it would be safer in the TARDIS. In April 1951, they moved a replica of the stone the Doctor had created to Arbroath Abbey, but it was accidentally dropped and broken in two. A group of Scottish nationalists took the blame for the theft, and the Doctor kept the real stone in his TARDIS, where it was content. (PROSE: Set in Stone)