In general, Key Stones are figures that come in sizes ranging from slightly smaller than Sonic's full height to about the size of a head. Each Key Stone have a square rock stand with a figure fixed on top of them, which is usually a lump of crystal, but also sometimes a statuette. All Key Stones found within a general area come in different varieties, with their own specific colors and shape. They can be found lying around in specific places in the game's environment.
Key Stones are used to unlock paths and doors found within the games that cannot be passed through by any other means. To unlock a path or door with a Key Stone, the player has to pick up a Key Stone and take it to a nearby pedestal with colors or symbols that matches that of the Key Stone, and then insert it there, which will then unlock the path or door. If a Key Stone is inserted into a pedestal that does not match it, nothing will happen. However, if inserted or just placed in the proximity of a matching pedestal, the Key Stone will automatically float up and insert itself into the pedestal.
Unlike conventional items in the Sonic the Hedgehog series, like Rings, regular keys or Emerald Shards, Key Stones do not disappear from the gameplay field and enter the player's in-game inventory once the playable character touches them. Instead, the player has to manually pick up the Key Stones through the game controls, and have the playable character carry it by hand during gameplay until they are released. While carrying a Key Stone, a playable character's speed is lowered and his/her unique actions cannot be performed (e.g. Sonic's Sonic the Hedgehog's Spin Dash, Tails's Flying, Sonic the Werehog's Skills etc.).
Sonic Adventure and Sonic Adventure DX: Director's Cut
Key Stones made their first appearance in Sonic Adventure and Sonic Adventure DX: Director's Cut. In these games, the Key Stones are used by the player to open up passages to the games' Action Stages in the Adventure Fields, allowing the player to play through new Action Stages and progress with the storylines. Key Stones can be found in the games' Adventure Fields and appear accordingly to when the player needs to enter the Action Stage that the Key Stone grants access to.
Sonic Adventure and Sonic Adventure DX: Director's Cut feature each a total of four different Key Stones:
In the version of Hot Shelter presented in Amy Rose's storyline, there are also objects called power cubes which works in the same way as Key Stone, and are used at two different points in the Action Stage. When the player manages to insert a set of four different colored power cubes into their matching fixture, it will unlock a new path for the player to continue down through.
Key Stones made another appearance exclusively on the PlayStation 3/Xbox 360 version of Sonic Unleashed. In this game, the Key Stones only appear in the Nighttime Action Stages where Sonic the Werehog is playable, and resemble crystals that come in the colors red, blue, green and yellow.
Here, the Key Stones are used to solve puzzles inside the Nighttime Action Stages by finding a certain number of Key Stones, which are hidden in the environment, and then inserting them into their matching pedestals, which will then open up doors or activate mechanics that will allow the player to proceed. While most doors open and mechanics activate with one or two of Key Stones, major doors will need all four Key Stones to open.