The Master Emerald is a large green Chaos Emerald that controls the power of the seven Chaos Emeralds. It can neutralize their power and make them useless unless someone uses their positive feelings.
It's early years are shrouded in mystery, but all that is known is that Knuckles became guardian of it and was born with guarding it.
Sonic Adventure 2Edit
Rouge the Bat tried to steal the Master Emerald, thinking was just a jewel. However, it was stolen by Dr. Eggman using his Eggmobile and Knuckles shattered it to keep it from getting stolen. The pieces were on the search by Knuckles and Rouge. Rouge found three in Dry Lagoon and Knuckles found three more in the Wild Canyon. Knuckles gathered up six more, three from Pumpkin Hill, and the other three in the Aquatic Mine. However, he lost them all when Sonic accidently opened the cockpit. Knuckles managed to retrieve them and took the pieces off Rouge, restoring the Master Emerald. He lastly used it to drain the Chaos Emeralds of their power to stop the Space Colony ARK.
The Master Emerald is only mentioned, when Rouge decides to find it after Eggman's secret treasure is revealed to be a room full of Shadow Androids.
It is only seen when Sonic tries to use it's power to calm the Chaos Emeralds inside Ultimate Emerl, but Emerl gets annoyed at it's light and destroys it. Knuckles somehow restores it prior to Sonic Rivals.
In these series, the Master Emerald exists every time, so when Eggman Nega turned the future Master Emerald into a card, every other Master Emerald disappeared as well. Knuckles eventually finds Eggman Nega and takes the Master Emerald card off him, using it to send Onyx Island to where it belonged in the future, restoring the original Master Emerald.
Sonic Rivals 2Edit
Eggman Nega steals the past Master Emerald, using it to power his Emerald Detector and it is taken by Rouge and Knuckles. However, they don't realize this until the detector is damaged in the Chaotic Inferno Zone. Rouge then steals it.
Sonic Chronicles: The Dark BrotherhoodEdit
The Master Emerald is stolen by Ix to move the Nocturne back into the real world. It is pulled to Metropolis by the Chaos Emeralds and stolen, and then taken into the Twilight Cage. It is used by Ix to transform into Super Ix and taken by Knuckles after he is defeated.
In Sonic X, it doesn't play a serious role except Knuckles trying to get home to return to guarding it until Chaos Control brings Angel Island into Chris' world and Knuckles returns to guarding it. However, Eggman destroys it, releasing Chaos. Knuckles restores it, and later it is revealed to have been the target of the Knuckles' Clan and they hurt Tikal and some Chao, whic drove Chaos to destroy the world.
It also plays a role in the Sonic Adventure 2 adaption, where it plays a similar role, only it is not shattered, only stolen by Rouge. Knuckles brings it with him when they return to their world and then left on Angel Island.
In Season 3, it plays a major role where Chris uses it to link with their worlds and travel there. It is later used to power the Blue Typhoon because of the Chaos Emeralds being scattered around the galaxy and the Sonic Power Cannon. In an attempt to destroy Dark Oak with all of it's power, it shatters and the power cannon is weakened. Dark Oak absorbs it's power and merges with Chaos, and Chris' particle data is corrupted, preventing him from returning to his world that way.