Kontron crystal

Kontron crystals were crystals found in some areas of the Time Vortex. The Borad used them to create the Timelash. The Sixth Doctor acquired two kontron crystals from the Timelash and used them to construct a device which could slightly alter time. He used it to reflect the Borad's weapon beam back to him, killing him. (TV: Timelash)

HG Wells was given a kontron crystal as a parting gift by Vena, which was later used by Vena to recall Wells to Karfel. (PROSE: Time and Again, Travers & Wells)

Edward Travers bore a ring that was family heirloom given to him by his father, which contained a crystal that was revealed to be a kontron crystal. It was, somehow, connected to the kontron crystal in the possession of Wells. The proximity of the crystals threw both Wells and Travers of course (Wells was heading towards Karfel, and Travers from 1969 to 1935), and they both ended up on a parallel Earth. (PROSE: Times Squared, Night of the Intelligence, Travers & Wells) The ring was eventually passed onto Anne Travers, who later gave it to Lucy Wilson in 2017. (PROSE: Avatars of the Intelligence)

While Amy Pond was fading out of existence, the Eleventh Doctor built a Chronon Blocker using a dalekanium coil and a kontron crystal spare around the Dalek Capital City, Kaalann. (GAME: City of the Daleks) The Doctor later used the same kontron crystal to build a tachyon feedback loop. (GAME: TARDIS)

Behind the scenes

The reference book The Universal Databank speculated that the Crystal of Kronos and the Enlightenment crystal were kontron crystals, and that kontron crystals were made of taranium.