White-Point Star

A White Point Star is a type of a diamond that is only found on the planet Gallifrey. The Time Lords, whilst being Time-locked in the Last Great Time War, sought a means to escape their imprisonment and thus their destruction. This led to Lord President Rassilon using 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 both the Time Lords and Gallifrey into the universe once more. It was destroyed by the Doctor with Wilfred Mott's gun. (DW: The End of Time)

It cannot be known whether the diamond was actually destroyed as a metal shell would not be able to destroy diamond, or the device was damaged so that the physical link was broken.