Key Stones
Key Stones are figures with rock type pedestals containing a lump of crystal or a statue on top. These are primarily used to unlock doors and additional paths otherwise inaccessible. In its debut in Sonic Adventure, the player must place the Key Stones on the corresponding pedestal to progress in the character's storyline. Other objects that functions the same way are called, Power Cores; rectangular cuboids with machinery inside only available in Hot Shelter of Amy Rose's storyline. Key Stones also appears on the PS3/Xbox360 version of Sonic Unleashed. In the game, these keys resemble crystals colored in red, blue, yellow, or green and only available in the Nighttime Stages. All four Key Stones are needed for the player to proceed.
List of Key Stones