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, so 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 through the Immortality Gate. The Tenth Doctor shot it with Wilfred Mott's gun, shattering the diamond and banishing Gallifrey and the Time Lords back to the last day of the Time War. (TV: The End of Time)

later claimed to have "stuffed Rassilon's face full of White-Point Stars." The resultant regeneration was extremely painful, so painful Ohila was forced to make Rassilon a special draught but he still screamed all the way through. (PROSE: Lords and Masters)

Behind the scenes
In TV: The End of Time, the white-point star was located in a small hole in Rassilon's staff. As of series 9 finale TV: Hell Bent, the staff had a different kind of ornament on top of it.