Pyrite Identification Guide

What Pyrite Looks Like

Pyrite (FeS2, iron disulfide) is the classic "fool's gold." It has a pale brass-yellow color, often with a slight greenish or grayish cast, and a bright metallic luster. It is opaque. Pyrite is famous for well-formed crystals: cubes (frequently with striated faces), pyritohedrons (12-sided), and octahedrons, in the cubic (isometric) system. It also occurs massive, granular, in radiating "pyrite suns," and as replacement fossils. Tarnish can produce iridescent or brownish films.

Step-by-Step Field ID Checklist

1. Note the color: brassy, pale yellow — cooler and paler than gold's rich buttery yellow.
2. Look for crystal shape: cubes with parallel striations on the faces are a giveaway. Striations on adjacent faces run at right angles.
3. Test hardness: pyrite is hard, 6–6.5; it will not scratch with a knife and can scratch glass.
4. Do the streak test: rub on unglazed porcelain. Pyrite gives a greenish-black to brownish-black streak — never gold.
5. Check brittleness: pyrite is brittle and shatters or powders; gold is malleable and dents.
6. Smell/note associations: may smell sulfurous when struck; sparks when hit with steel.

Key Diagnostic Tests

• Hardness: 6–6.5 (Mohs) — far harder than gold (2.5–3) or chalcopyrite (3.5–4).
• Streak: greenish-black to brownish-black.
• Specific gravity: ~5.0 — heavy, but much lighter than gold (~19).
• Fracture: conchoidal to uneven; brittle.
• Crystal habit: striated cubes, pyritohedra, octahedra.
• No reaction to dilute HCl; not magnetic (unless altered to pyrrhotite/magnetite).

Common Look-Alikes and How to Tell Them Apart

• Gold: gold is deeper yellow, very soft (dents and cuts with a knife), malleable (not brittle), extremely heavy (SG ~19), and leaves a yellow streak. Pyrite is hard, brittle, lighter, and streaks dark. This is the key distinction.
• Chalcopyrite: more golden and often iridescent-tarnished, but much softer (3.5–4, knife scratches it) with a greenish-black streak. Pyrite resists the knife.
• Marcasite: same chemistry (FeS2) but orthorhombic, paler, more prone to crumbling and forming whitish sulfate coatings; lacks pyrite's good cubes.
• Gold-colored mica (biotite/phlogopite): flexible flakes, very soft, not metallic-hard.

Where Pyrite Is Found

Pyrite is one of the most widespread sulfides on Earth. It occurs in hydrothermal veins, sedimentary rocks (shales, coal), metamorphic rocks, and igneous rocks worldwide. Notable crystal localities include Spain (Navajún, with superb cubes), Peru, Italy (Elba), and many U.S. mining districts. Look for it in sulfide ore bodies, coal seams, and quartz veins.