Kontron crystal

Kontron crystals are crystals found in some areas of the Time Vortex. The Borad used them to create the Timelash. The 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. (DW: Timelash)

REF: The Universal Databank speculated that the Kronos Crystal and the Enlightenment crystal was a Kontron crystal, and that Kontron crystals were made of taranium.