Leptonite was an element that came in the form of crystals.
Leptonite crystals were once used by the Seventh Doctor and Mel to defeat the Quarks onboard the space yacht Pinto.
The Doctor claimed that if Quarks came within half a mile of a leptonite crystal, they would "go berserk and explode". He further noted that the only planet in the universe that produced leptonite was Puxatornee. Thus, they had to leave the Pinto mid-battle in order to retrieve some of the crystal from the planet. In the process, they became embroiled in an entirely different adventure. (AUDIO: Flip-Flop)