Pyrite

Pyrite occurs as a common, widely distributed mineral with a brassy, metallic yellow luster and often a distinctive cubic or striated crystal form, found in association with many other ores and readily mistaken, by color alone, for native gold, a confusion common enough among inexperienced prospectors and miners across history to give the mineral its enduring popular name, fool's gold. Struck sharply against steel, flint or another piece of pyrite, it throws a shower of sparks hot enough to ignite tinder, a practical property that made pyrite, alongside flint, a genuine everyday firestarting material for a very long stretch of human history, and that gave the mineral its Greek derived name, from pyr, fire.

Alchemists took a real interest in pyrite for reasons beyond its resemblance to gold: weathered pyrite, exposed to air and moisture, oxidizes naturally into a green vitriol crust, meaning pyrite deposits were themselves a natural source of the iron vitriol so heavily used across the workshop, and observing this transformation directly connected the mineral world to the vitriol chemistry alchemists practiced deliberately. Pyrite was also examined, and sometimes deliberately roasted, as a source of sulfur and of iron compounds more generally, its abundance making it a convenient raw material even where a purer or more specific mineral might have been preferred in principle.

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.