The Master Emerald is a massive, green emerald and a mystical relic of incredible power that keeps Angel Island afloat. The gem has existed since before any recorded history, leaving it's origins shrouded in mystery. It is located on Shrine Isle, and is protected by Knuckles the Echidna.
The Master Emerald has existed since well before historic record keeping had began. Where the gem came from is shrouded in mystery, though some have said that it was created by the Gods to neutralize the power of the seven Chaos Emeralds in the event that anybody attempted to use them for evil. Roughly 4,000 years ago, the ancient Knuckles Tribe, an Echidna clan led by Pachacamac, residing in what is known today as the Mystic Ruins, attempted to seize the gem to obtain ultimate power. The gem was, at that point, located at a shrine on the outskirts of their village, where a colony of Chao played peacefully, guarded by the mystical water beast Chaos. During this time, Pachamac's daughter, Tikal, discovered the shrine and the Chao colony herself and, when the time came to raid, Tikal stood between them, begging her people to leave the creatures, and the Emerald, in peace. However her pleas fell on deaf ears, the tribe forced their way past the Chao, prompting their guardian, Chaos, to appear and wipe out most of the tribe in a fit of rage. The creature out of control, Tikal prayed to the emerald to calm Chaos down, in which the gem absorbed both Chaos and Tikal into itself, in the process continuing to surge energy into a large part of the continent, causing it to break away from the rest and float into the sky with the remaining echidnas, creating Angel Island. (VG: SA/SADX)
Sought by Eggman
In the present day, Angel Island and the Master Emerald remains guarded by one remaining echidna, Knuckles the Echidna. Spending a solitary life alone in his duty, the emerald came under threat when the Death Egg crashed onto the island, weighing it down back into the ocean. Knuckles encountered the ships creator and pilot, Dr. Eggman, who convinced the somewhat guidable echidna that Sonic the Hedgehog and Miles "Tails" Prower, both on the doctors tail, where on their way to the island to steal the Master Emerald. As Knuckles went on to confront the duo upon their arrival and proceeded to make their journey to bring down Eggman considerably more difficult, the doctor took the opportunity to steal the Master Emerald himself as a power source to relaunch the Death Egg, much to Knuckles' humiliation at realizing he's been tricked. With Knuckles' help, Sonic managed to get on board the Death Egg as it launched, leading to a final battle in outer space between Eggman and Super Sonic. Eggman's final robot was destroyed, and Sonic returned to the surface, Master Emerald in hand, and returned it to Angel Island, allowing it to arise once again. (VG: StH3, S&K)
Some time later, the Master Emerald once again fell into Eggman's sights: after learning of the legendary water beast Chaos, the Doctor sent his forces to smash the Emerald into pieces, releasing the monster from its limbo. With the Emerald's power no longer in place, Angel Island once again fell into the ocean. The Island in danger, Knuckles initiated his first quest to hunt down the missing Emerald shards that had flown off everywhere. After a long search, Knuckles restored the Emerald to its former glory, and soon Chaos was even returned to its home within it, now recovered from the rage it had felt since 4,000 years prior. (VG: SA/SADX)It didn't take long before the Emerald was once again put in danger, this time by a more unlikely suspect: a jewel thief named Rouge the Bat. As the two fought over possession of the gem, Eggman turned up to swipe it from under their noses in hopes of using it in his latest plan. However Knuckles made the ultimate sacrifice by once again shattering it before Eggman could escape. Angered at losing her newest treasure, Rouge vowed to hunt down the scatter pieces for herself before Knuckles could. Once all the pieces had finally been returned aboard the Space Colony ARK, the restored Emerald became key to stopping the Chaos Emerald-fueled space station from hurtling toward the Earth. (VG: SA2/SA2B)
Shattered World, Shattered EmeraldDuring the Shattered World Crisis, the Master Emerald became the object of conflict between three opposing sides. Shadow the Hedgehog of Team Dark were sent by G.U.N. to recover the emerald and keep it in a secured location, while Knuckles stood his ground in guarding its location. Meantime, Shadow's new rival Eclipse the Darkling, who crash-landed on the island, sought the emerald's power to help him and the remaining Dark Arms conquer their enemies. In the chaos of it all, having fought both Shadow and Eclipse, Knuckles recited Tikal's Prayer and shattered the emerald as a last resort. He then left the island in order to search for the shards and restore the Emerald before the Freedom Fighters restored the planet and caused Angel Island to fall into the ocean. (SU: #67, #68, #69, #70)
Accompanied by Chip, Knuckles began his search and eventually arrived in Casino Park, where they recovered an Emerald shard from a woman who had made it into a necklace and claimed that it was a family heirloom. He subsequently teamed up with the Knothole Freedom Fighters and made plans to find the shards with help from Amy Rose and her Mystic Melody. The pair quickly obtained a shard in Apotos. At one point, Sonic located another shard beneath the Green Hill Zone, but this event was erased from history due to a paradox. Unfortunately, another fragment later fell into the hands of Walter Naugus, restoring at least some of his lost magical abilities and making him and his allies-Wendy Naugus and her Witchcarters-an even greater threat. Meanwhile, Knuckles and Amy found another shard on their way to Moss the Sloth's hut in the Rocky Jungle Zone. While staying at the hut, Amy learned that the remaining shards were clustered into groups. (StH: #269, #272, #276, #281; SU: #76)
The Master Emerald contains extremely potent and raw energy which can be harnessed to power machinery or potentially initiate a super transformation, and can greatly increased innate talents. Like the Chaos Emeralds, its power can also be harnessed to perform certain reality-warping feats.
As its primary function, the Master Emerald contains special powers that lets it nullify and control the Chaos Emeralds' energies. This grants it the ability to either completely shut down the Chaos Emeralds' chaos energy or potentially amplify their power. However, it seems that only those affiliated with the Knuckles Clan, such as Knuckles and Tikal, are able to use the full potential of the Master Emerald. To unlock its power, they must on most occasions recite Tikal's Prayer.
If the Master Emerald is shattered into several pieces, it can repair itself if its shards are brought back together. However, shattering the Master Emerald seems to give the shards an explosive trajectory, causing them to fly a great distance which makes the task of gathering them rather painstaking.