A Mega Stone is a type of held item that can be held by a compatible Pokémon to Mega Evolve in battle. Each Mega Evolution requires a different Mega Stone to be used.
A Mega Stone can be obtained by finding and defeating a Mega Evolved boss Pokémon, who will drop its corresponding Mega Stone upon defeat. If the Mega Stone has not already been obtained from a boss Pokémon before, it will always be dropped by the boss Pokémon; otherwise, it will have a 140 chance of being dropped.
A Pokémon can Mega Evolve in battle only if it is holding its corresponding Mega Stone and if its holder is wearing a Mega Bracelet.
In battle, a Mega Stone is not affected by moves that remove held items or held item effects, such as Trick, Knock Off, Embargo, and Magic Room.