The Dark Crystals are dark jewels radiating with dark magical energy and they served as the power source for the dangerous Golem known as the Destroyer, and apparently the Golems of the Deep as well. They are featured in The Legend of Spyro: Dawn of the Dragon.
In a few levels of The Legend of Spyro: Dawn of the Dragon, destroying these crystals is a mandatory action.
In the Floating Islands, on 2 occasions, the player must destroy Dark Crystals that are growing on torches. Unlike any other type of crystals, the Dark Crystals have the ability to steal breath power from the player when they come close enough to them.
Hunter told Spyro and Cynder in the Catacombs that these minerals radiated with black magic and were never natural, and suggested that they should destroy the crystals from the Golem's dislodged arm quickly. When standing right beside them, strange eerie noises and what seems to be creepy, ghastly voices can be heard eminating from the crystals. This could be a product of the dark magic from these crystals or possibly evil spirits.