In the Anime
Each Jewelpet has their own Jewel Charm and is considered their "sleeping" state after the magicians cast a special spell on them. The Jewelpets stay on that form until they are awakened by either Rinko or Dian.
The Jewel Charm is a symbol of a Jewelpet's bond between a Jewelpet and their human partner and is used as a magical device by the people chosen by their Jewelpet partners. The caster needs to say "twinkle, twinkle, magical charm, winkle, winkle, jewel flash!" to activate the magical powers while they also change the caster's costume in the process to increase its magical potential.
Jewel Charms are where all the Jewelpets originally came from. After casting magic, the Jewelpet must remain on their Jewel Charm form to recharge their energy.
The setting of Jewel Charms being where Jewelpets came from was remained. Jewelpets turn back into their Jewel Charm form when they are severely injured.
Jewel Charms made a brief appearance in the first episode, but after that they never appeared nor were mentioned.
Jewel Charms were replaced with normal-looking gems. A Jewelpet returns to this form when they are severely injured or sealing themself.
Jewel Charms made a brief appearance in the beginning of the opening sequence. They also appeared in the first episode in the form of a book, but then they never appeared nor were mentioned.
- So far, Larimar is the only canon Jewelpet who has yet to have her Jewel Charm revealed.