Crystals are large blue minerals (not to be mistaken for their glowing counterparts).
On Credit Island, when gems are found underground, you will find a soul that will go to the Crystal.
Crystals are either found in massive formations in the world or dug out from deep below, in the game however it is unknown how these crystals will be found. They may also be linked with the mysterious Corruptrons.
In the Prototype, if you place an barrel with crystals in it near the main spawn crystal, Bricktrons will spawn when sufficient souls have been created. This is also the case with the bags of material that the mining Bricktrons carry. If they contain crystal, it will be removed from the bag when walking near the spawn crystal.