Captain Bravo mentioning about the Philosopher's Stone for Kazuki.
The Philosopher's Stone is a concept briefly mentioned in the Buso Renkin series, due to much of the story material dealing with the pseudo-science of alchemy. As Alchemy in the Buso Renkin universe is essentially the same as real-world Alchemy —transmutation of matter, artificial humans etc.— the Philosopher's Stone is touched upon during a conversation regarding the Alchemic Warrior organization and alchemy's backstory, but receives no further attention.
It is explained that there are indeed efforts to create the Philosopher's Stone in progress, but it is far from complete. The Kakugane were created due to some of these efforts.