- You may be looking for the individual from The Ultimate Adventure.
Crystals had a great variety of valuable (and in some cases, strange) uses.
Crystals with unusual properties
Radiation and/or luminiscence
Time travel and time manipulation
Xion crystals were one component of the Doctor's TARDIS. When they got misaligned, this caused it to go off course. (COMIC: Pinball Wizard) In place of a time rotor, the Thirteenth Doctor's control console had a large crystal that glowed orange while in flight. (TV: The Ghost Monument)
Certain crystals could focus and/or harness psychic energy. The people of Manussa used the Great Mind's Eye, a crystal artificially created in zero gravity, to accidentally create the demonic Mara. The Little Mind's Eye destroyed the Mara. (TV: Snakedance)
The Xylok resembled crystals. They valued purpose more than anything else. Stored in computer form, they were capable of communicating with human computer software, storing vast amounts of information, accessing databases, hacking teleports, containing beings, defending itself and many, many more functions. One such Xylok was known as Mr Smith and worked with Sarah Jane Smith to defend Earth. (TV: Invasion of the Bane et al.)
The Spherions were a species of crystals, reproducing by incubating their young in organic matter. Unaware that it was possible for organics to be sentient, the Spherions used the Calaxi as incubators following their homeworld being colonised. When they crossed paths with the Eighth Doctor and Josie Day, the two relayed to them that organics were living beings, with the Doctor finding them a new planet. (COMIC: Music of the Spherions)
Other uses for crystal
The Doctor's sonic screwdriver used a crystal, similar in composition to the ones from Metebelis III. The Tenth Doctor ejected it from his screwdriver to draw a handful of Eight Legs away from himself and Martha Jones. (COMIC: The Forgotten)
Behind the scenes
According to the audio play adaptation Seven Keys to Doomsday, the Crystal of All Power, capable of good or great evil, destroyed the Masters of the planet Karn. Consequently, the Masters broke the crystal into seven small crystals scattered across the planet, which the Daleks sought to implement their Doomsday Plan. The Doctor also sought the pieces. This wiki considers neither Seven Keys nor the original play, Doctor Who and the Daleks in Seven Keys to Doomsday, a valid source.