A quantum crystalliser was a Time Lord device designed to recognise certain possible outcomes as either positive or negative, and to eliminate all timelines so that only the ones considered positive to the user remained.
The Celestial Intervention Agency once placed such a device in one of Hulbert Logistics' battle machine to help ensure they defeated the Cybermen on Lonsis. Lucie Miller found it due to her time sensitivity from having travelled in time. The Eighth Doctor disapproved of its use, but he later expanded its reach to the whole machine, killing all Cybermen on board. This action destroyed the quantum crystalliser. (AUDIO: Human Resources)