The Aquarius Stone is a key item found and used in Aqua Rock in Golden Sun: The Lost Age. Its description reads "The stone that leads the chosen one." 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, and retrieving it is essentially one of your "main objectives" throughout exploring the entire interior portion of Aqua Rock. At the bottom of the same screen, accessible only by entering the room from the bottom, is an altar which you are to place the Aquarius Stone onto. The wall of water to your immediate north will be parted by magic, creating essentially a walkable "hallway" that leads you to the final room of Aqua Rock, where the stone tablet that bestows the Parch Psynergy to Piers is located.