Cup of Athelstan

The Cup of Athelstan was a historical treasure stored in the International Gallery in London, worth 18 million pounds. It was made of solid gold.

History
The cup was given to Athelstan, King of England, in 924 AD as a coronation gift from Hywel, King of the Welsh.

Lady Christina de Souza stole it during Easter 2009 and then travelled on a bus, the 200, through a wormhole to another planet. The Tenth Doctor later used it to get the bus and its passengers back through the wormhole to Earth. The gold was conductive and able to be the basis for an electrical current. Unfortunately, he had to destroy the cup in the process because it did not fit into the apparatus constructed to house it, forcing the Doctor to bash it down to size with a hammer. Christina was chastened at the loss of the antique. (TV: Planet of the Dead)

After the Cup of Athelstan's disappearance, security was vastly improved for the Mona Lisa. (TV: Mona Lisa's Revenge)