The Guardian Stone is a mystical gem located in North's Workshop. The Man in the Moon summons it when he is showing the Guardians their newest member.
The Guardian Stone is only used when the Man in the Moon is choosing a new guardian. The gem used the Man in the Moon's moonlight to refract the light and created a spectrum of the new Guardian.
The Guardians were supersized to see the stone and were exited to know who will be the newest member: the member that can help them to stop Pitch Black. Bunnymund wasn't happy with the idea of Jack Frost becoming a Guardian as the stone showed an image of the trouble making Winter Spirit, but North trusts in Manny's choice and he knew that Manny wouldn't have chosen Jack if he didn't think or believe that he could help them.
- The gem is located in North's Workshop under a floor design that can only be open by the Man in the Moon's moonlight.
- In The Art of Rise of the Guardians, a sketch of the "Altar Design" reveals that the stone is made out of moon rock, indicating the stone was a gift from The Man in the Moon, containing part of itself (in the film, MiM is a spiritual deity), so that is can directly make contact with the Guardians.