The Dark Gem is very mysterious if not totally understood. It was a difficult capture because it was able to repel solo attacks. Dark's powers are to engulf an area in darkness, create black portals leading to the realm of darkness, or act like Amazon and create streams of black ribbons. It takes on the form of a black panther.
Appearing in Episode 7, it caused the night to remain during the morning, which also caused everyone with non magical powers who were asleep, to continue sleeping. (Madison noticed because she couldn't sleep that night). While searching, it's dark portals captured everyone but Kero and Ceaira. When they found the panther, Kero transformed and attacked it while Ceaira combined the gems of Firey and Watery to stop it. When summoned it has the same powers as it's unsealed form.