Jewel Charms (ジュエルチャーム, Jueru Chāmu) are the jewelry that Jewelpets turn into in their down-time. The specifics of how they work, their utility and their sizes vary from anime season to anime season.
In Jewelpet, each Jewelpet has its 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.
In Jewelpet Twinkle☆, the Jewel Charm is a symbol of a Jewelpet's bond between a Jewelpet and its 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 it also changes the caster's costume in the process to increase its magical potential.
In Jewelpet Sunshine, the Jewel Charm is where all the Jewelpets originally came from. After casting magic, the Jewelpet must remain on its Jewel Charm form to recharge their energy.
In Jewelpet Kira☆Deco!, the setting of Jewel Charms being where Jewelpets came from was remained. Jewelpets turn back to their Jewel Charm form when they are severely injured.
In Lady Jewelpet, Jewel Charms were replaced with normal gems, when a Jewelpet is severely injured or sealing itself.
In Jewelpet Magical Change, 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.