One of the mighty emeralds in the
Sonic the Hedgehog series. The Master Emerald has the power to neutralize the power of the
Chaos Emeralds. It is large, green, and perfectly shaped as if it had been cut by a craftsman.
Knuckles the
Echidna is its sworn protector, guarding it from whoever would steal it (usually
Dr. Robotnik).
The Master Emerald was first seen on Angel Island in Sonic the Hedgehog 3 (and its continuation, Sonic and Knuckles/Sonic 3 and Knuckles) when Robotnik stole it and convinced Knuckles that Sonic the Hedgehog was the enemy. Dr. Robotnik planned to use the emerald to power his latest grand scheme, but before he could launch his evil plan Sonic and Knuckles settled their differences and teamed up, defeating Eggman and taking back the emerald.
In Sonic Adventure (and Sonic Adventure International as well as Sonic Adventure DX) the Master Emerald was shattered when the evil Chaos escaped from its gemstone prison. Also released at this time was Tikal, an ancient echidna who had been sealed within the emerald with Chaos to keep his evilness in check. Eventually Sonic the Hedgehog defeated Chaos and returned him and Tikal to the emerald, which Knuckles the Echidna had repaired.
The Master Emerald is seen again in Sonic Adventure 2 (and its Nintendo GameCube port, Sonic Adventure 2 Battle) when Dr. Robotnik steals the emerald again. Knuckles shatters the emerald to keep it from being taken away and then has to go and collect the pieces of the emerald to restore it. However, Rouge and Bat is also after the emerald and is also collecting the pieces. Eventually they team up and Knuckles is able to restore the emerald. Sonic and friends then use it to cancel out the awesome power of the Robotnik's space laser weapon (which is powered by the seven Chaos Emeralds).
Will the Master Emerald appear again? Time will tell...