Smalls figures that unlocks doors and paths when placed in their matching pedestals.
Key Stones are objects that first appeared in Sonic Adventure. They are small figures that work as keys, which can be picked up by the playable characters and be used by the player to open sealed doors or paths by placing them in their matching pedestals.
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 that cannot be open by any means. To unlock such paths with a Key Stone, the player has to pick up a Key Stone and take it to a nearby pedestal with colors or symbols matching that of the Key Stone, and then insert it there, which will then unlock the path. If a Key Stone is inserted into a non-matching pedestal, 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 various collectibles, Key Stones do not enter the in-game inventory once the playable character touches them. Instead, the player has to have the playing character manually pick it up and carry it by hand during gameplay until they are released. While carrying a Key Stone, a playable character's speed is lowered and their unique actions cannot be performed (e.g. Sonic's Sonic the Hedgehog's Spin Dash, Tails's Propeller Flight, 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 Amy Rose's version of Hot Shelter there are objects called power cubes which works in the same way as Key Stones and are used at two points in the Action Stage. When the player inserts a set of four different colored power cubes into their matching fixture, it will unlock a new path.
Sonic Adventure 2
In Sonic Adventure 2 and Sonic Adventure 2: Battle, a variant of the Key Stones called the Gate Key appears in Pyramid Cave. These Key Stones are used to open Gate Key Doors which block paths that the player needs to proceed down through. One Gate Key is needed for each door and they are always located in the nearby vicinity.
Here, the Key Stones are used to solve puzzles inside the Action Stages by finding a certain number of hidden Key Stones 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 need one or two of Key Stones, major doors will need four in total.