The Master Emerald is a large, green emerald depicted in the Sonic the Hedgehog video game series. It is often used as a plot device throughout the series- mostly Sonic Adventure and Sonic & Knuckles. The guardian of the Master Emerald is none other than Knuckles The Echidna.
 Storylines involving the Emerald
 Sonic & Knuckles
The Master Emerald first appeared in Sonic & Knuckles. The emerald was taken from its shrine within the Hidden Palace by Dr. Eggman in his bid to take over the world, and upon obtaining it, he decided to use it to restore power to the Death Egg. Additionally, it was used to give Mecha Sonic a respective "Super" state. However, Sonic, Tails and Knuckles fought to take it back and it was eventually returned to Angel Island.
 Sonic Adventure
In Sonic Adventure, it was shown keeping Angel Island afloat. The emerald also held within it the ancient monster known as Chaos. In the game, it was shattered by a mysterious force, causing Angel Island to fall and Knuckles having to find the pieces to put it back together.
 Sonic Adventure 2 (Battle)
Sonic Adventure 2 shows Knuckles fighting with Rouge for possession of it. Eggman tried to steal it again while they were distracted, but Knuckles voluntarily shattered the Emerald in order to keep it out of his hands. Knuckles and Rouge then set off separately to gather the shards of the emerald for their own reasons. In Sonic Adventure 2 Battle: Knuckles uses the Master Emerald to try and stop the power of the Chaos Emeralds.
The Master Emerald was formed when Mammoth Mogul absorbed the powers of eleven chaos emeralds from Enerjak. He used this power to become Master Mogul. However his plot was thwarted by Sonic, Tails, Knuckles, and the Ancient Walkers. They then utilized the powers Mogul absorbed and created the Master Emerald. Since one of the emeralds Mogul absorbed had been used to keep Angel Island afloat the newly created Master Emerald was used on the island instead.