Metebelis crystal

The Metebelis crystals were blue sapphires from the planet Metebelis III.

Properties
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)

History
The Third Doctor visited Metebelis III and took a crystal back with him. It was no small task, requiring him to brave several dangers, making the trek alone. However, he succeeded in finding a beautiful sapphire. He was able to use it break the hypnotic control BOSS had put Mike Yates under. Unfortunately, Brigadier Lethbridge-Stewart was also entranced by the crystal and became unresponsive, requiring the annoyed Doctor to snap him out of it. The Doctor let Yates use the crystal to infiltrate Global Chemicals, where he released James from conditioning in the same manner he had been freed. Finally, the Doctor used it on Stevens, which cut him loose from BOSS's link to his mind. Stevens then sacrificed himself to overload BOSS and make the computer explode, also destroying the Global Chemicals Research Centre. 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)

In 1974, the Eleventh Doctor used a Metebelis crystal to enhance Emma Grayling's psychic abilities in order to enter a pocket universe. (TV: Hide)

After the Eight Legs went back to 2015 to conquer Earth, they were able to cover the planet in crystals, as well as using them to control the disciples of the Eight Truth. (AUDIO: Worldwide Web)

The Doctor made a psionometric isolator powered by a Metebelis crystal. (PROSE: The Quantum Archangel)

The Master once used a group of Sontarans to try and steal the Doctor's crystal. (GAME: Destiny of the Doctors)

Metebelis crystals were sold at Primo Temp. (COMIC: Buying Time)