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This crystal seems to mess with a Pokemon's DNA. It allows the form to show a trait from a previous evolution. It curiously also works on Trainers.

The mutation crystal allows for a trait to be passed over from a previous evolution OR the appearance of a form that isn’t evo/gimmick-based. This can allow a Jigglypuff to take on the form of a ScreamTail for example but Pokemon in these altered forms cannot evolve out of them. For example, a stage 2 Screamtail can’t evolve into a Wigglytuff but the other side of the fusion could still evolve.