Spirit Stone

The Spirit Stones (霊石) are mysterious items that appeared in Kamen Rider Kuuga (manga) manga.

Maternal
The Maternal (母体) is a monolith size Spirit Stone that can turn human into the Gurongi or Agito.

Amadam
One of the Spirit Stone is the Amadam (アマダム), which is the power source of Kamen Rider Kuuga.

Alter Ring
Another derivation of Spirit Stone is the Alter Ring (オルタリング) that provides energy to Kamen Rider Agito.