Dark Crystals are a sub-weapon used by Gabriel Belmont. The crystals are pure Shadow Magic, they are a natural phenomenon that grow in areas with a high concentration of Shadow energy. Because the crystals are made of magic in itself, activating magic of any kind will not unlock additional abilities or effects. The crystal can be found either in shards (common) or as a whole crystal (rare) in the environment or from enemy drops. Four shards are needed to make a whole crystal.
Exactly how is unknown, but crushing one of these crystals will summon a powerful shadow demon that will utterly devastate enemies. Due to the degree of power held within them, Gabriel can only carry one Dark Crystal at a time.