The clue is in the name... The "philosopher's" stone is not a real substance at all, it is a spiritual and intellectual ideal, a state of being; akin to, but not the same as, Buddhist elightenment. Alchemists disguised their gnosticism behind the rather complex sounding chemical processes, and knew that very few would be able to see beyond their personal greed (talk of gold and elixiirs of life makes people very greedy) to the spiritual secrets.