The Ultragem is a jewel capable of turning its holder's thoughts into power, much like the Chaos Emeralds. The gem lies in Castle Grimm, a landmark of Planet Gremlyn, and is protected by the guardian Serpentus. Criminal Xenor Krell travelled to the castle to make the Ultragem's powers her own, but was defeated by Serpentus. The arrival of Amy Rose and Tekno the Canary gave her a second chance, so she tricked Amy into traversing the treacherous castle herself. Amy located the gem in a well-hidden chamber, allowing Krell to unleash her true attentions.
Xenor Krell utilised the Ultragem's powers in several ways. She first ensured Amy and Tekno couldn't interfere by blasting them with a poweful beam. She then emitted a bright green light before turning the floor Amy stood on to quicksand. Creating a wall of flames disturbed Serpentus once more, but Krell used the Ultragem once more to turn the snake into mist. Amy realises its powers and convinces Krell to change the walls of the castle into solid silver. She then insults the thief, which brings Krell to show her the same fate as Serpentus. As the hedgehog dodged the blast, it reflected off the polished walls and turned Krell herself into mist. The Ultragem also got caught in the blast, meaning nobody could ever abuse its powers again.
- The Ultragem goes from orange, to green, to blue, to red with no explanation.