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Sulfide · Disulfide (S₂ pairs) · Pyrite Group

Pyrite

FeS₂

Fool’s gold: brassy, striated cubes that spark when struck, hide real gold, and weather into rust and acid.

Hardness6–6.5
Crystal systemCubic
LusterMetallic
Sp. gravity4.95–5.1
ClassSulfide
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Pyrite specimen

Brass-yellow pyrite cubes with characteristic striated faces — ‘fool’s gold’ — Photo: TheFurther21 · CC BY-SA 4.0

What is Pyrite?

Pyrite is iron disulfide — FeS₂ — the glittering ‘fool’s gold’ that has embarrassed prospectors for centuries. The joke cuts both ways, though: real gold is frequently found inside pyrite.

It is the most common sulfide mineral on Earth, growing as brassy cubes scored with fine striations, as twelve-sided pyritohedra, or as masses and raspberry-textured framboids in veins, coals and black shales.

Strike it against steel and pyrite throws sparks — the Greek pyr, ‘fire’, gave the mineral its name and helped arm the wheel-lock guns of the 16th century.

In one sentence: Brass-yellow metallic cubes, often with striated faces; hard (6–6.5), brittle and heavy — but far lighter than gold, and it leaves a greenish-black streak.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Striated cubes. Look closely at a cube: the faces carry fine grooves, and the striations on neighboring faces run at right angles. That cross-hatched pattern is pyrite’s fingerprint, produced as two crystal forms competed during growth.

Hardness and streak. At 6–6.5 pyrite scratches glass and laughs at a pocket knife (gold, at 2.5–3, dents). Dragged across unglazed porcelain it leaves a greenish-black streak; gold leaves a yellow one.

Brittleness. Hit pyrite and it shatters — or sparks. Hit gold and it flattens. Malleability, not color, is the real test.

Tarnish and decay. Fresh pyrite is pale brass; weathered surfaces go iridescent or rusty brown. In damp collections it can oxidize into sulfuric acid that eats its own specimen label — the notorious pyrite decay.

How & Where It Forms

Pyrite forms wherever iron meets reduced sulfur: in hydrothermal veins with gold and copper ores, as an accessory mineral in igneous and metamorphic rocks, and above all in oxygen-starved muds, where bacteria generate hydrogen sulfide that locks iron into tiny framboids in coal and black shale.

Its chemical twin — same formula, different structure — is marcasite, which grows in cooler, near-surface settings and weathers even faster.

Often grows alongside:

QuartzChalcopyriteSphaleriteGalenaCalciteArsenopyriteGold

What It’s Used For

Sulfuric acidRoasted pyrite was long a leading source of sulfur and sulfuric acid — the world’s most-produced industrial chemical.
Sulfur oreMined on a massive scale at places like Rio Tinto, mainly for sulfur with iron as a bonus.
Collectors & ‘marcasite’ jewelrySharp cubes are specimen staples; the glittering ‘marcasite’ of Victorian jewelry is actually cut pyrite.
Gold indicatorReal gold often occurs as tiny inclusions inside pyrite — find pyrite, check for gold.

How to Identify It

The fool’s-gold test is simple: streak. Pyrite leaves greenish-black on unglazed porcelain; gold leaves gold. Pyrite is brittle and hard; gold is soft and malleable. Chalcopyrite is softer and brassier (a knife scratches it), pyrrhotite is magnetic, and marcasite is paler and tin-white.

Greenish-black streak on unglazed porcelain (gold leaves yellow)Hard (6–6.5) and brittle — it shatters, never dents or flattensBrass-yellow cubes with striated faces; striations run at right angles on adjacent facesHeavy for its size (SG ≈ 5), but nowhere near gold’s 19.3
Looks like…But differs because…Quick test
GoldSoft and malleable with a yellow streak and SG ≈ 19 — pyrite is brittle with a black streakStreak & dent test
ChalcopyriteSofter (3.5–4), deeper brassy color, and often iridescent tarnishA knife scratches chalcopyrite
MarcasiteSame FeS₂ formula but orthorhombic; paler, tin-white, and far less stableColor & crystal shape
PyrrhotiteBronze-brown, softer, and usually magnetic — pyrite is notMagnet
BorniteMuch softer (3) with a purple-blue ‘peacock’ tarnishTarnish & hardness

Where It’s Found

Notable localities for fine specimens:

Navajún, Spain Rio Tinto, Spain Rio Marina, Italy Huanzala Mine, Peru Quiruvilca Mine, Peru Dalnegorsk, RussiaOpen the locality map →

References & Further Reading

  1. Dana’s New Mineralogy, 8th ed. Disulfides, pyrite group.
  2. Mindat.org — Pyrite mineral data and locality records.
  3. Wikipedia — Pyrite.

Last reviewed 2026-09-14 · How to cite this page.