White-Point Star

A White Point Star was a type of a diamond 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 to escape into the universe. However, whilst the link was present in his mind, there was no physical link. Rassilon sent a small White Point Star Diamond through the time-lock to establish one. The Master recovered it and used it to bring the Time Lords and Gallifrey into the universe once more. It was shot at 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.