|“||When the noise subsides, I walk up to the place where Kraid had once been. It was how I expected. A small black stone is rolling around in the center of the spot where Kraid exploded. And this egg-sized, oddly-shaped, jet-black rock is the Key Stone.||„|
Canonically, in Metroid and Metroid: Zero Mission, the entrance to Tourian can only be accessed by the two Stone Statues, which are activated by defeating Kraid and Ridley. The Golden Statues in Super Metroid operate nearly identically, requiring the defeats of Kraid, Ridley, Phantoon, and Draygon. In Metroid, Samus Aran has to shoot the activated statues with her beam in order to generate the bridge to Tourian; in Zero Mission and Super Metroid, the statues automatically react to her presence in the room, and she does not directly interact with them. None of these games show Samus actually using keys or any other objects to activate the statues.
In Zebes Invasion Order, an explanation is provided for why defeating the Space Pirate leaders causes the Stone Statues to activate. Kraid and Ridley each keep a Key Stone, which is described as resembling an egg-sized oval-shaped black stone. When either Mini-Boss is killed, he explodes in a bright flash, only leaving behind the Key Stone on the ground. Samus automatically collects the Key Stone before leaving the room.
In room B-65, Samus finds a table with what appears to be a Key Stone resting on it. However, this stone is a plastic forgery; the real Key Stone is still held by Ridley. If Samus tries to take the fake Key Stone or gets too close to it, she is trapped in the room and is forced to use Bombs or Missiles to escape before she can fight Ridley.
Once a Key Stone has been obtained, Samus can insert it into a slot located at the base of its respective Mini-Boss's Stone Statue, causing the statue to rise and trigger a mechanism that opens a lock. Both Key Stones are required to open both locks and open the gate to Tourian.