What It Actually Is

Pyrite is identified as iron disulfide, FeS2, an entirely uncontested modern identification and one of the more chemically simple minerals discussed in this atlas. Its resemblance to gold is purely visual and has no basis in any shared chemistry between iron and gold, two elements with almost nothing in common beyond both forming solid, somewhat lustrous minerals; the popular test of scratching a suspect gold sample, since pyrite is notably more brittle and harder than soft, malleable gold and streaks a dark greenish-black powder rather than gold's own metallic yellow streak, was a reliable, empirically sound way for pre-modern assayers to distinguish the two without any chemistry beyond careful observation.

Pyrite's genuine chemical connection to the alchemical workshop runs not through gold at all but through iron and sulfur, the same natural weathering process that turns pyrite to rust-brown powder in open air being chemically the same oxidation that produces green vitriol from iron sulfide minerals more broadly. Pyrite's abundance and its striking crystal habit have also given it a long afterlife well outside chemistry, as a common mineral specimen and decorative stone, an ordinary use with no real historical connection to its alchemical or metallurgical interest.