The Mindstone is a piece of crystal that connects the Knights of Crystallia together. It is a shard from the Worldspire. It connects all of the Crystin together, allowing them to share abilities. That means that if one of them knows how to do something, each of the rest of them get a tiny bit better at that thing.  The specifics of the Mindstone are implied to not be known to the general population outside of the Crystin.