The Apocalypse Gem is a blood-red ruby that is embedded in the forehead of Doctor Apocalypse. Its origin is unknown.
The gem apparently serves as a portal to an alternate, demon-infested dimension.
Doctor Apocalypse is able to use the gem to borrow super powers from the demons in the other dimension. With concentration, he is able to customize his super-powers suite to include seemingly any powers that he desires. It is uncertain at this time what the limits on the gem’s powers are.
Following the Doctor’s capture, MEDUSA imprisoned him and strapped a Neural Dampener on him in order to neutralize his powers. Without Apocalypse’s ability to keep the portal closed, demons were able to come through the portal.
It also appears that the gem is able to be removed from the Doctor’s head. While examining the unconscious Doctor, the hero Messenger began to extract the gem without any difficulty (the remainder of the Eclipse stopped him from removing the gem)