The ShadowGems & The Rules Of The Power Gems.

The ShadowGems are made from a materiel from Limbo, the native name is unknown, but most refer to it as Limbite. 5 were originally made to kill the Demi-God of death, Heric, but Heric fused with 1 of the ShadowGems, so the old Legends used the remaining 4 to seal him away, There are currently  only 3 due to Corruption being made of the Corrupt ShadowGem. Only one Universe has the ShadowGems, Dimension 1, The Gems of Power (Any powerful gemstone made of Limbite), have certain rules, here they are. The current Power Gem holders are the Legends, 

Vane: X ShadowGem

Falcon-I: Nega ShadowGem

Blitz: LuminoGem

Sam: Custom Gem

Dragoneye: Mega ShadowGem

Heric: ShadowGem

Corruption: Corrupt ShadowGem

1. You can't have more than 8 in a dimension at 1 time or else it will crumble like a cookie.
2. If one is destroyed all of the Exoverse will be destroyed (Even Custom) 
3. They have a certain user chosen, no one else can hold them besides the users.
4. Only one of each Gem can exist in the Multiverse.