The Great Seal of Diplos. (TV: The Stones of Blood)
The Great Seal of Diplos was the official seal of the Diplosian government. It had the powers of transmutation, transformation and the establishing of hyperspatial and temporal coordinates. It was stolen by Cessair when she fled the planet. On Earth, she wore it as a necklace.
In actuality, the Seal was a segment of the Key to Time. Upon its recovery by the Fourth Doctor and Romana I, it was returned to its original state. (TV: The Stones of Blood)
