The Philosopher's Stone is an extremely rare gemstone from throughout the Star Ocean series. It is often used to create each character's Ultimate Weapon (as far as I.C. is concerned) (For example Faize's Onyx saber).
It is possible to create it using alchemy on both Star Ocean: Fantastic Space Odyssey/First Departure and Star Ocean: Second Story/Evolution.
It can be made in Alchemy in Star Ocean: Till the End of Time, and it is required to recruit Ansala.
In Star Ocean: The Last Hope, it is obtainable in the second post-game dungeon, the Wandering Dungeon.