The Metebelis Crystals were blue crystals from the planet Metebelis III.
Metebelis Crystals could break hypnotic spells, enhance intelligence and enhance psychic powers. (TV: The Green Death) 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. This radiation could be blocked by minerals also found on Metebelis III. (TV: Planet of the Spiders) Metebelis Crystals were the only source of fluon radiation. (PROSE: Return of the Spiders)
The Third Doctor visited Metebelis III and took a crystal back with him. He was able to use it break the hypnotic control BOSS had put Mike Yates under. The Doctor later gave the crystal to Jo Grant as a wedding present. (TV: The Green Death)
In the far future, spiders from Earth were accidentally brought to Metebelis III. They were mutated by the Metebelis Crystals, developing into the Eight Legs. The Great One, the leader of the Eight Legs, tried to increase her mental powers to infinity by using a web of the crystals. The spiders went back in time to Earth to take back the crystal that the Doctor had received back from Jo Grant. Once the web 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. (TV: Planet of the Spiders)