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)
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.