Master Emerald

The Master Emerald is a large gem from Sonic's dimension that has existed for thousands of years; it resides atop an altar, guarded by Knuckles the Echidna, and is connected with the seven chaos emeralds, as it is equally mysterious. This emerald contains infinite energy, possessing the ability to both empower and negate the chaos emeralds, should their energy need to be enhanced or discontinued.

During the War Above Earth in "Dimension Mix-Up," Zim, GIR, and Minimoose abruptly come across the Master Emerald while aboard the Death Egg. Upon finding the team beaten by Metal Sonic, who has claimed all seven chaos emeralds, Minimoose throws the emerald to GIR, who absorbs its power and causes his own super transformation; he easily overpowers Super Mecha Sonic, most likely negating the chaos emeralds' energy, as Super Mecha Sonic now appears helpless against GIR's exceptionally rapid attacks. After liberating the chaos emeralds from Metal Sonic's grasp and destroying him afterward, GIR releases the Master Emerald, and Knuckles initially grows furious, assuming that they had stolen it. However, it becomes clear that Doctor Eggman had stolen the Master Emerald while the team was in space, and they later confront him in the Death Egg's control room; during the hostage situation, Sonic is forced into handing both the Master Emerald and the chaos emeralds over to the Doctor, but suddenly, Shadow appears and takes them all back, before kicking the Doctor across the room. Later, Mario uses the Master Emerald, along with his own Fire Flower, to create a significantly large fireball and blast Bowser out of the Death Egg and into deep space. Finally, Shadow and the E-Series robots use the Master Emerald and the chaos emeralds to push the descending Death Egg out of Earth's atmosphere, and then cause Chaos Control to send both themselves and the station away from the planet; all of the emeralds then fall back down to the surface, where the group is standing.

Afterward, the Master Emerald remains atop the altar on Angel Island where it belongs, even as Gemerl steals the chaos emeralds during "Dimension Takeover," and while Calvin and Hobbes investigate their disappearance later on.