Master Emerald

For the Sonic the Hedgehog video game series' version of the Master Emerald, then look up the gemstone of the same name, on the "Sonic News Network" wiki page.
The Master Emerald is a large green gem of ultimate power with the ability to turn thoughts into reality. Those who harnesses it will be granted the ability to assemble and destroy civilizations. The most powerful weapon in the universe, the Master Emerald was forged ages ago by the echidna race, with all seven Chaos Emeralds.