Red Stone

Though merely one of the various monikers attributed to the Philosopher's Stone in the manga storyline, in the 2003 anime storyline, a Red Stone is a separate, artificially synthesized alchemical amplifier modeled after the legendary Philosopher's Stone. Much like the Philosopher's Stone, the Red Stone allows alchemists to transmute without the aid of a Transmutation Circle, bypass the law of Equivalent Exchange in transmutation and even has the power to grant advanced alchemical skills to non-practitioners of alchemy. However, as an imperfect compound created by mundane methods, Red Stones lack the power of the true Philosopher's Stone and become unstable after a seemingly random number of uses, causing them to break apart mid-transmutation and run the risk of triggering an alchemical Rebound. Easily mass-produced, the Red Stones act as the primary mode of nutrition for Homunculi and are handed out to alchemists whose abilities and forbidden ambitions the Homunculi intend to exploit. In the 2003 anime, when within a special seal, Homunculi have been known to vomit Red Stones from their body such as what happened with Greed, Sloth and Lust.

Red Water
Unlike the methods by which Philosopher's Stones are created, Red Stones are manufactured by compressing large amounts of Red Water into a single crystalline form. This crimson fluid is an extremely toxic yet powerful mixture of other alchemical amplification substances and can easily be produced in large quantities. Despite its high toxicity, it was discovered by the alchemist Nash Tringham that the most efficient and effective method of crystallizing Red Water is to introduce it into an existing biological process like the human body. Fortunately, his sons Fletcher and Russell later discovered that a similarly efficient crystallization occurs when Red Water is introduced to plant life.

It is likely, given the attributes of the Red Stone and Red Water, that they are modeled after Cinnabar.

Trivia

 * The concept of imitation Red Stones was derived from Edward Elric's conclusion that Father Cornello's Philosopher's Stone was a fake after seeing it run out of power and disintegrate. In the manga storyline, this is simply an incorrect assumption: Cornello's Stone had simply used up all its souls, and as Ed had yet to learn of the Stones' true origin, he initially believed the legend and assumed their energy was supposed to be infinite.