Summon Gems

Summon Gems (召喚石) are special items that appear in Kingdom Hearts. They each contain the heart of a creature that had escaped its homeworld's destruction. They can be awakened by the Fairy Godmother at the Magician's Study at Traverse Town, unlocking the characters as summons when done so. The four gems contain the hearts of Simba, Dumbo, Bambi, and Mushu. The only summons that are not obtained from a gem are  Genie and Tinker Bell, who are unlocked after sealing  Agrabah's and Neverland's Keyholes, respectively.

In Kingdom Hearts Chain of Memories, Kingdom Hearts II, and Kingdom Hearts III, summon gems are replaced by summon cards, summon charms, and heartbinders respectively.

Kingdom Hearts
After the Heartless were unleashed and devoured countless worlds, a handful of hearts escape destruction of their individual homeworlds by living on as summon gems. Simba is capable of doing so owing to his powerful heart; Dumbo due to his strong, pure soul; Bambi thanks to his innocence; and Mushu because of his determination.

The Summon Gems are scattered across the worlds, but Sora retrieves them all during his travels. Once awakened by the Fairy Godmother, those contained in the gems assist Sora in his journey. Following Ansem's defeat, the hearts return to their own worlds.