Great Seal of Diplos

Great Seal of Diplos was the former official seal of the Diplosian government. It has the powers of transmutation, transformation and the establishing of hyperspacial 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 Doctor and Romana, it was returned to its original state. (DW: The Stones of Blood)