White-Point Star

A White Point Star was a type of a diamond that was found only on the planet Gallifrey. The Time Lords, whilst Time-locked in the Last Great Time War, sought a means to escape their imprisonment and their destruction. Lord President Rassilon used the Master as a means of escaping into the universe. However, whilst the link was present in his mind, there was no physical link. Rassilon was able to send a small White Point Star Diamond through the time-lock to establish one. The Master recovered it and used it as a means of bringing the Time Lords and Gallifrey(the literal planet) into the universe once more. It was destroyed by the Tenth Doctor with Wilfred Mott's gun. (DW: The End of Time)

It is not known whether the diamond was actually destroyed, or if the device was damaged so that the physical link was broken.