The emerald seems to contain lots of energy, as shown by the fact that it can trigger a form change on its own. It has the ability to merge with other people, enhancing their abilities. It also appears to have a mind of its own, choosing who it merges with, speaking telepathically to those who merge with it.
Nickolas the Hedgehog "Did you really need to add that!" Nick get outta here!
Lunari the Echidna "Why, I'm honored." Lunari, I know you can break the fourth wall telepathically. Quit showing off.