Aquarius Stone

The Aquarius Stone is a key item found and used in Aqua Rock in Golden Sun: The Lost Age. In the massive interior portion of the dungeon, the largest "hub" room has this item in a chest at the top of its central structure. At the bottom of the room is an altar which you put the stone onto, and it will cause the nearby wall of water to magically part in two to reveal a path to the dungeon's goal, which is the room where the Parch Psynergy is learned.