Crystals are completely impassable and give no terrain stars. They cover one square and become plains tiles after destruction, although there is clearly rubble left over. Black Hole units within two spaces of the Crystal get 2HP restored, along with their fuel and ammo, for free. Along with the Black Obelisks, they are the only way to repair an Oozium.
They act as the main source of Black Hole's power in Dual Strike and were invented by Lash for the Bolt Guard's campaign in Omega Land. They produced massive amounts of energy and materials, but turned the surrounding area into desert and wasteland.
Appearances as a mission objective
The first time a Crystal appears is in the two-front mission Victory or Death!, where Koal uses one to bolster his forces. It is protected by Lash's Black Arc, which drops bombs to impede the progress of the Allied Nations.
Finally, three Black Crystals appear in Means to an End. They act as the power for a barrier protecting the weak spots of Von Bolt's gargantuan Oozium, the Grand Bolt, and are protected by Kindle's forces.