Key Stone

Key Stones are objects that appear in Sonic Adventure, Sonic Adventure DX: Director's Cut and the PlayStation 3/Xbox 360 version of Sonic Unleashed. 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.

Description
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, which the player can pick up with the playable character.

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
Where they were first introduced, the game had four key stones in total (and four more devices that work like key-stones in Hot Shelter).

Wind Stone
The Wind Stone shows up just outside Tails' Workshop in the Mystic Ruins after the fight with the Egg Hornet. It is green with a swirling symbol within it.

The Wind Stone is to be taken to the cave just nearby in the Mystic Ruins with wind blowing through it and placed at the small alter where to put the Wind Stone, the entrance to Windy Valley will open.

Ice Stone
The Ice Stone shows up just after Casinopolis. (In DX it is hinted to be dropped by Cream the Rabbit). The Ice Stone is located in the alleyway behind the cinema near the casino. It is cyan with a snowflake symbol within it.

The Ice Stone is to be taken to Mystic Ruins, where a tunnel to Angel Island has been revealed. When you go up to the tunnel to the island there is a cave surrounded by ice and the small alter where to put the Ice Stone. Inside the frozen cave is a pool of water, (Big's Life Belt) and a ladder leading up to Ice Cap.

Gold & Silver statues
"Two stone statues, a door to the past"

- Hint-orb.

In Knuckles story he will need to find two keystones, gold and silver, in the form of statues to enter Lost World. The silver statue is found on the cliff near the exit of the jungle and the gold one is found under a mound in a cave with yellow indicating circles that Knuckles must dog for. They are to be taken behind the pyramid at the center of the jungle to open the back entrance to Lost World.

Hot Shelter
For Amy to proceed in Act 2 of Hot Shelter, Amy enters a room full of monitors. In the center of the room are four colored squares (purple, yellow, blue and orange) and scattered around the room are colored glass like cubes, each of the colors in the center of the room. When Amy puts the blocks in the right holes, ZERO will crash the monitors which will make a way out for Amy. While these cubes are not magical key-stones and perhaps function by technology, they still function the same way.

The same puzzle will follow afterwards but more challenging as ZERO will be harassing you and some of the boxes are hidden.

Sonic Unleashed
In Sonic Unleashed, the Key Stones return exclusively for the stages at night as Sonic the Werehog can pick up objects. The stones (which look a little worn down than their predecessors) are mostly used in puzzle solving and come in colours red, green, yellow and blue. While some doors open and mechanics activate with one or two of these stones, major doors will need all these crystals to open.