The Philosopher's Stone is a stone sought by alchemists for its ability to turn lead and other basic metals into Gold. Gideon Cahill was seeking it when he created the Master Serum. In The Sword Thief, Amy and Dan think that it is what the Clues add up to before they find it's true power, but alas, they're wrong. The Philosopher's Stone is similar to the Master Serum. The symbol of the Philosopher's Stone can be seen at The Jefferson Puzzle above the door of the burning house.