A Key Stone (キーストーン, Key Stone) is a stone that acts as a medium between a Pokémon Trainer and a Pokémon holding a Mega Stone in order to make them Mega Evolve. A Key Stone is typically embedded in a personal accessory. When the Key Stone is activated, the Pokémon's corresponding Mega Stone reacts to it, starting the process.
A Key Stone can be inserted in various objects so that it can be kept on the owner's person. Many characters use different objects to hold their Key Stones.