The Master Emerald is a powerful gemstone in the Sonic the Hedgehog video game series. It is known for its ability to control the energy of the Chaos Emeralds, which are smaller gems that grant various abilities. The Master Emerald is often depicted as a large, green crystal and is typically located on Angel Island, a floating landmass. In the lore of the series, the Master Emerald is protected by Knuckles the Echidna, a character who is the guardian of the island. The emerald plays a crucial role in maintaining balance and stability in the world, often being sought after by villains like Dr. Robotnik for its immense power.