Ultragem is an object that appears in the Sonic the Comic series published by Fleetway Editions. It is a gem that was hidden in Castle Grimm on the Planet Gremlyn and has the strange power of transforming the molecules of anything it touches to its holder's whim.
Xenor Krell tried to steal the Ultragem, but only managed to do so thanks to Amy Rose and Tekno the Canary, whom she duped into believing that she needed it for the common good. Xenor Krell used the Ultragem to defeat Serpentus, then transformed the castle into solid silver. When she turned on Amy Rose, the silver castle reflected the Ultragem’s rays and both Xenor and the gem were reduced to vapor.