Pre-Super Genesis Wave Timeline

Summary

The Master Emerald is a magical emerald that is protected by Knuckles the Echidna. It has enough power to match all of the 7 Chaos Emeralds. Knuckles is very protective of the emerald and was born for this purpose. But he often falls asleep on the job. He rarely ever lets anyone touch it.

Uses 

The Master Emerald was once used in Sonic X as an alternative to the 7 Chaos Emeralds used to power the spaceship Tails made, titled the Blue Typhoon. It was used to go into space to defeat the Metarex.