It descibes a group of con-men cheating various people of their money by pretending to discover the Philosopher's Stone. This was believed to bring untold riches, since it turned all metals into gold. One of the victims is called Sir Epicure Mammon, and he is imagining what life will be like after he has the Philosopher's Stone. My meat shall come in, in Indian shells, Dishes of agate set in gold , and studded With emeralds , sapphires , hyancinths , and rubies . It seems less muddled than Oscar Wilde's list .
Carbuncle is certainly garnet here, as rubies are mentioned separately.
We still have jacinth mentioned, and it can't be sapphire, because that is named explicitly. Sir Epicure Mammon doesn't mention amethyst .