Crystal

Crystals had a great variety of valuable (and in some cases, strange) uses.

Radiation and/or luminiscence
The planet of Metebelis III was known for blue crystals which emanated radiation. The crystals mutated spiders from Earth into the intelligent and telepathic Eight Legs. (TV: Planet of the Spiders)

Radioactive Hymetusite crystals powered both Skonnan and Nimon technology. The Skonnan Empire demanded tribute of these crystals from the people of Aneth. (TV: The Horns of Nimon)

More trivially, Terileptils used vintaric crystals for lighting. (TV: The Visitation)

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)

The Crystal of Kronos enabled to create TOMTIT and to summon the eponymous Kronos. (TV: The Time Monster)

The Borad used kontron crystals to create the Timelash. (TV: Timelash)

Communication
The Black Guardian gave crystals to Vislor Turlough and Menlove Stokes to direct them in his plans against the Doctor. (TV: Mawdryn Undead, PROSE: The Well-Mannered War)

Psychic energy
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 Great One, leader of the Eight Legs, for this reason sought the Great Crystal which the Third Doctor stole from her planet. (TV: Planet of the Spiders)

Unspecified powers
The Eternity Crystal was a crystal of immense power created by the Darksmith Collective for the Krashoks which was capable of reanimating life. (PROSE: The Dust of Ages-The End of Time)

Other uses for crystal
The Crystal Bucephalus, a restaurant, was so-named because of its sculpture of the legendary horse of the same name. (PROSE: The Crystal Bucephalus)

Lady Cornelia Basildon-Stone removed the roof on a palace in 1894 and replaced it with crystal. (COMIC: The Crystal Throne)

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 Fourth Doctor, or a version played by Trevor Martin, 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.