Mega Stone

A Mega Stone is a type of held item that allows Pokémon to Mega Evolve.

Mechanics
During a battle, a Trainer can use their Key Stone (held in the Mega Ring) to Mega Evolve a Pokémon that holds its corresponding Mega Stone before it attacks. Pokémon only remain Mega Evolved during battles and Pokémon Contest Spectaculars, and will revert to their original state once the battle has ended or if they faint. Every Pokémon that can undergo Mega Evolution has a corresponding Mega Stone that is named after it.

Mega Stones cannot be transferred to or from the correct holder during battle: Thief and Covet do not take the Mega Stone, Knock Off does not remove the Mega Stone, Trick and Switcheroo fail, Fling and Bestow fail, and Symbiosis does not activate. Embargo and Magic Room also cannot prevent the use of Mega Stones to Mega Evolve Pokémon. Mega Stones are not consumed upon use, but only one Pokémon can Mega Evolve per battle.