The Philosopher's Stone (哲人核, Tensujinkaku; literally, "Philosopher Stone"), also known by various other names such as the Red Stone, the Fifth Element, and more, is a powerful transmutation amplifier appearing in the anime, the manga, and this fanon. Many of the main events in the canon series revolve around the Elric brothers' attempts to obtain this stone, in order to gain back their original bodies.
Generally believed to exist only in legend, the Philosopher's Stone has long been sought by alchemists as the ultimate goal of their craft. While alchemy is an amazing preternatural art in and of itself, its scientific limitations are such that the dream of a mystical and mythical object capable of eradicating those limits is one shared by generations of practitioners. Though, in more pragmatic times, the legend of the Philosopher's Stone has come to exist only in the fanciful illusions of Romantics, the fabled amplifier is, in fact, an arcane reality.
The Philosopher's Stone can take many forms, ranging from a lumpy, coal-like rock to a viscous liquid, as with the one that Dr. Marcoh owned, and their color in the purely material form is always a dark, blood-like red. It can greatly boost the effectiveness of an alchemist's transmutation, allowing them to bypass the Law of Equivalent Exchange and perform more powerful transmutations than that which would normally be within their ability. Also, it is rumored that the Philosopher's Stone can allow one to complete a human transmutation. While it is stated that the Philosopher's stone allows one to 'break' the Law of Equivalent Exchange, in reality, the price is already paid, as to allow one to do something incredibly difficult (such as bring back the dead), countless human lives are sacrificed in order to create the stone. Each life or soul is priceless in alchemical terms, so it allows an alchemist to exchange these souls for something else. What they are exchanged for is completely up to the alchemist himself. However, it can range from the completely equal resurrection of another human, to the worthless repairing of an inanimate object.
The raw material of Philosopher's Stone is a very large amount of human lives, ranging from dozens to approximately over fifty million. The souls of humans are compressed transmuted into a stone, leaving the spirit and body as by-products. The first stone in history was transmuted by "Father", using the souls of the people of Xerxes. This event inspired the legend of a country which was destroyed in a single night. The later Philosopher's Stones were transmuted by Doctor Tim Marcoh and his fellow alchemists who were instructed to make the Stones by Lust and the other Homunculi, using the lives of Ishvalan prisoners, and later the makers themselves to cover it up. This series of Philosopher's Stones wasn't complete - they were made from only about 13 prisoners. In the manga, the stone that Father Cornello used was also one of this series.