Key Stone

Key Stones are crystals that unlike conventional items like rings, keys or emeralds, are too large to simply "pocket" and need to be carried by hand during gameplay, thus hinder the characters speed and abilities. These crystals usually have specific colours and fixed on a square rock stand. They need to be taken to special small alters with matching colours or symbols where the key-stones will fit in order to open a sealed door or path.

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 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.

Two stone statues
"Two stone statues, a door to the past"

- Hint-orb.

In Knuckles story, he will need to enter Lost World, to do so he will need to explore the jungles of Mystic Ruins to find two small square ruins filled with dirt. He will need to dig dig to get to the statue buried in the dirt. The ruins with yellow circles over it indicate the gold snake statue is buried and the ruins with the white circles indicate the silver snake statue. 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 coloured squares (purple, yellow, blue and orange) and scattered around the room are coloured glass like cubes, each of the colours 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 harrasing you and some of the boxes are hidden.

Sonic Unleashed
In Sonic Unleashed, the Key Stones return exclusivly 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.