Keystones are bones that merged with planets, to foster each planet's development. Every life-bearing planet has a bone that serves as its keystone. In other words, those keystones are like the support columns of every planet. However, if a planet loses their keystone bone, it disappears.
Known keystone bone users:-
- Griffon Bone - Keystone of unknown planet destroyed by Revolt in Episode 25, it's adept is Dark Griffon who's real name is unknown.
- Pheonix Bone - Keystone of Neop Angelis. It's adept is Stolz
- Dragon Bone - Keystone of Earth, it's adept is Shougo Ryuujin