If held by an Metagross, allows it to Mega Evolve into Mega Metagross if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Metagross a Metagrossite or remove a Metagrossite from Metagross.
Outside of battle
If held by an Metagross, allows it to Mega Evolve into Mega Metagross when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Metagross will remain a Mega Metagross until the Talent Round ends.
