A Mega Stone is a special type of item in the Pokémon video game series that allows certain Pokémon to undergo a transformation known as Mega Evolution. When a Pokémon holds its corresponding Mega Stone during battle, it can temporarily evolve into a more powerful form, gaining enhanced stats and sometimes new abilities or types. Each Mega Stone is unique to a specific Pokémon species, such as Mewtwo or Gardevoir. To use a Mega Stone, a player must have a Pokémon that is capable of Mega Evolution and must also have a Mega Ring or similar item to trigger the transformation during battle.