Crystal (The Ultimate Adventure)

Crystal 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. (DW: Planet of the Spiders)

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

Time travel and time manipulation
Xion crystals were one component of the TARDIS. When they got misaligned, this caused it to go off course. (Pinball Wizard)

The Great Crystal of Kronos enabled the Master to create the TOMTIT machine and to summon the eponymous Kronos. (DW: The Time Monster) The Borad used kontron crystals to create the Timelash. (DW: Timelash) Kontron crysals

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. (DW: Snakedance)

The Great One, leader of the Eight Legs, for this reason sought the Great Cystal which the Doctor stole from her planet of Metebelis Three. (DW: Planet of the Spiders)

Unspecified powers
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, respectively, also sought the pieces. (TP: Doctor Who and the Daleks in the Seven Kes to Doomsday)