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The Master Emerald is an item in the Sonic series. It is the supervisor for the Chaos Emeralds, meaning that any damage to it needs to be amended as soon as possible or the results will be disasterous (an example being the collapse of Angel Island at the start of Sonic Adventure). Invariably it is guarded by Knuckles, usually in isolation (the most notable exception to this rule is Sonic Heroes 2).
The Master Emerald, while ancient, acts as a counter against the Chaos Emeralds, nullifying it's power if need be. It holds up Angel Island. Knuckles the Echidna acts as the sole protector of it. Only those from the Knuckles Clan can truly use it's full potential. It has the same powers as the seven Chaos Emeralds as well, allowing users to power up into a Super form.
The Master Emerald is also somewhat unique in terms of it's placement in the space time continuum. It exists simultaneously in all time lines across the space time continuum. Should the Master Emerald disappear in one time frame, it would cause a chain reaction that would cause it to disappear across all the time frames in the space time continuum.
In this game, it is guarded by Shade and Knuckles, owing almost entirely to the obnoxious infatuation of both parties towards each other in that game.
Super Mario Silver Spurs Trilogy
Super Mario and Friends: Zennu Quest
Here Evil count Dorkula tries to find this Emerald to use its power and conquer Zennu, Mushroom World and Real world in same time.