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Whenever you right click it gives you a level back, up to the maximum possible amount stored. It is useful for preserving your precious Experience.
Currently there is a bug which causes it to not show returned levels when you right click the Mind Stone. This is a bug and can be fixed by either by a relog, or switching between dimensions (End, Nether, Overworld) the result being your levels are returned. It can also be fixed by picking up XP orbs, in some cases.