The Key Stone is a very unique item that is used in conjunction with Mega Stones to help certain Pokemon achieve a Mega Evolution when in battle. There is only one Key Stone that can be obtained within this game - since only one is needed to help Mega Evolve all the Pokemon that can - but players won't be able to get their hands on it until later in the game. In order to obtain the Key Stone, players must first obtain seven Gym Badges and then find that the Viridian City Gym remains locked. From here, your Rival will suggest going to see Professor Oak to check if he can help. When you go back to the Pokemon Lab in Pallet Town, Blue will be there with Professor Oak. It is from Blue at this point in the game when you will obtain the Key Stone.