Black crystals,also known as dark crystals are physical manifestations of dark energy that can warp, twist and/or corrupt surrounding objects and organic matter, especially young dragons, who are more saceptable. however, if the corrupted one be master of dark powers, such as Cynder and Malefor, they can bend the powers of these crystals to their will, however, if they fail to do so, the crystals can cause dragons to turn against one another (see Dark Cynder and dark silver) such events started the first dragon civil war 800 years ago.
The Crystals are first mentioned to originate from meteors, though not much detail is given to them.
The Legend of Cynder: Atlantean Rising
They appear to be native to Casperion, a moon just within the Dragon Homeworld's gravitational field.
The Legend of Cynder: The Precursor Legacy
During the Dragon Civil War, the novels state that Cynder has to eat the crystals in order to be able to use elemental attacks, though how the crystals physically affect her, or if they do at all, is not discussed.