philosophers' stone vs philosopher's stone

philosophers' stone and philosopher's stone both are nouns.

Word	Noun	Adjective	Verb	Adverb
philosophers' stone	Yes	No	No	No
philosopher's stone	Yes	No	No	No

As nouns, philosophers' stone and philosopher's stone are synonyms defined as:

philosophers' stone and philosopher's stone: hypothetical substance that the alchemists believed to be capable of changing base metals into gold

Other synonyms of philosophers' stone include elixir.

philosophers' stone (noun)	philosopher's stone (noun)
hypothetical substance that the alchemists believed to be capable of changing base metals into gold	hypothetical substance that the alchemists believed to be capable of changing base metals into gold