Master emerald
The master emerald is what makes Angel island fly it can also be used to power the death egg during the events of sonic 3 and knuckles. It also is the resting place of chaos and Tikal the echidna.
The master emerald it was used to power angel island and since chaos has been freed it has been shattered.
Master Emerald
A Magical Gemstone guarded by Knuckles the Echidna that gives people Super Forms/Super States/Super Shields when used. Can be used to control the Chaos and Super Emeralds. However, Knuckles never uses it's power as it's his job to protect it. Not use it. It also keeps Angel Island in the sky.
Knuckles: I'm gonna go to Angel Island to check if the Master Emerald hasn't been stolen by Rouge, Eggman, or anyone else.