Proteus crystals are solidified forms of Protomatter once used by the Xel'Naga to alter planets to support life.
Composed primarily of Xenorium, Alkahel 44, and trace amounts of other substances charged with psionic energies, Proteus crystals are extremely unstable. Upon contact with any form of matter, the crystals alter the molecular structure to match whatever psionic information they were imprinted with. If sufficiently charged and left unchecked, the crystals would continue to assimilate and alter all nearby matter (Command & Conquer: Tiberium).
Due to the unstable nature of the crystals, any impact or stimulation can cause them to explode with tremendous force (Beast Wars: Unstable Energon).