Information in this article may not be canonical to the storyline of the games or any other Sonic continuity.
Ultragem is an object that appears in the series published by Sonic the Comic 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. 
Color Blocks | Chaos Emeralds ( Extreme Gear) | Crystals | Emeralds | Emerald Shards | Forcejewels | Fake Emerald | Gems ( Blue, Green, Purple, Rainbow, Red, Sky, Yellow) | Great Emeralds | Master Emerald | Phantom Ruby | Phantom Ruby prototype | Precioustone | Sol Emeralds | Super Emeralds | Time Stones
Sonic the Comic Black Emerald | Grey Emerald | Chaos Emeralds | Master Emerald | Soul Crystal | Ultragem
Sonic the Hedgehog Deep Power Stone | Energy Crystal | Power Rock | Time Stones
Adventures of Sonic the Hedgehog Chaos Emeralds ( Immortality, Invincibility, Invisibility, Life, Ring)
Archie Comics Pre-SGW
Anarchy Beryls | Ancient Onyx | Chaos Emeralds ( Dark Mobius, In Another Time, In Another Place) | Freedom Emeralds | Guiding Star Gem | Master Emerald ( Dark Mobius) | Master Precioustone | Sol Emeralds | Super Emerald | Time Stone
Chaos Emeralds | Emerald Shards | Master Emerald | Sol Emeralds | Time Stones
Sonic Underground Chaos Emerald | Power Stone
Sonic X Chaos Emeralds | Master Emerald
Sonic Boom Chaos Crystals | Crystal Shards | Fregosi Sapphire | Lost Crystal of Power | Meroke Crystal | Tummel Crystal
IDW Publishing Chaos Emeralds | Master Emerald | Sol Emeralds | Warp Topaz