Kyfred Gem
From Tardis Wiki, the free Doctor Who reference
Kyfred Gems were highly valuable crystals found only on Gallifrey. According to the Twelfth Doctor, they were considered the rarest gems in the universe.
After the Twelfth Doctor's TARDIS became stranded in the Panopticon during a temporal crisis, K9 Mark IV detected a nearby cache of Kyfred Gems and the Doctor decided to "borrow" them, hoping that the Time Lords would not mind.
Although he originally intended to use them to buy other equipment, the Doctor quickly realised he could use the Gems as temporal "skipping stones" to restabilise the temporal energy of the continuum and headed to the Untempered Schism with Bill Potts to attempt it. (GAME: Lost in Time)
