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The Spirit Stones (霊石, Reiseki) are mysterious items that appeared in Kamen Rider Kuuga manga.
Maternal[]
The Maternal (母体, Botai) is a monolith size Spirit Stone that can turn human into the Gurongi or Agito.
Yukina Tsugami was captured by Maternal's tentacles.
Incomplete transformations usually made the Failed Agito.
Amadam[]

- Main article: Amadam
One of the Spirit Stones is the Amadam (アマダム, Amadamu), which is the power source of Kamen Rider Kuuga.
Any given successors can inherit the Amadam from the previous host if they are willing. However, the successor will suffer great pain and see illusions when merging with it.
Altering[]

- Main article: Altering
Another derivation of Spirit Stone is the Altering (オルタリング, Oruta Ringu) that provides energy to Kamen Rider Agito.
If its host died, the Altering is able to transplant itself automatically to the user's nearest relatives.
Notes[]
- "Reiseki" (霊石 ) is also the title of the 19th episode of Kamen Rider Kuuga.