Metebelis crystal
The Metebelis Crystals were blue crystals from the planet Metebelis III. They could break hypnotic spells, enhance intelligence and enhance psychic powers as well.
The crystals were blue due to the planet's blue moon and were found in deep caves in the planet. Metebelis crystals created radiation when they were grouped together. (DW: The Green Death)
The Great One, the leader of the Eight Legs, tried to increase her mental powers to infinity by using a web of the crystals. Once it was completed with a perfect crystal the Third Doctor had brought her, the web overwhelmed her and she was destroyed. Due to the radiation of the caves, the Doctor regenerated into his fourth incarnation. (DW: Planet of the Spiders)
The Master once used a group of Sontarans to try and steal it. (VG: Destiny of the Doctors)
