A sulfide with style

Pyrite is FeS₂, iron disulfide, in the isometric crystal system. Its brassy metallic color and remarkable habit diversity make it a staple in collections—from razor‑sharp cubes to complex twinned clusters.

“Fool’s gold” and actual fire

The nickname comes from prospectors mistaking pyrite’s glitter for gold. But pyrite’s Greek‑rooted name (pyr, fire) tells another story: strike it and you can produce sparks—ancient fire‑making the mineral way.

Hydrothermal veins

Pyrite precipitates from hot, sulfur‑bearing fluids in quartz veins and cavities, often with galena, sphalerite, chalcopyrite, calcite, fluorite—and sometimes precious metals.

Sedimentary & diagenetic

In oxygen‑poor muds and shales, iron and sulfur from decaying organics form framboidal pyrite (raspberry‑like clusters of microcrystals). In some black shales, it grows as flattened “pyrite suns” (discs) along bedding planes.

Igneous & metamorphic

Pyrite occurs as disseminations in igneous rocks and recrystallized grains in metamorphics. It’s ubiquitous because iron and sulfur are, too.

Weathering tale

Exposed to air and moisture, pyrite can oxidize to iron oxides/hydroxides (limonite/goethite) and sulfate minerals. In bulk rock, this reaction drives acid mine drainage; in specimens, it can cause surface tarnish or “pyrite disease” if humidity stays high.

Associations

Common companions: quartz, calcite, galena, sphalerite, chalcopyrite, arsenopyrite, fluorite. In lapis lazuli, fine pyrite flecks create a starry‑sky effect.

Why it sparkles so well

Fresh crystal faces are highly reflective. Striations on cube faces act like tiny diffraction gratings, giving a “banded sheen” as you tilt the piece under light.

Palette & luster

• Brass‑yellow — fresh surfaces, classic “golden” glow.
• Bronzy — subtle darkening with age or micro‑tarnish.
• Iridescent films — thin oxidation rainbows on some surfaces.

Luster is metallic; broken surfaces may show uneven to conchoidal fracture with bright, granular flashes.

Habit words

• Cubes (striated) — fine parallel lines on faces (pyrite’s autograph).
• Pyritohedron — 12 pentagonal faces (unique to pyrite), often with subtle curvature.
• Combinations — cube + pyritohedron; occasional octahedral modifications.
• Framboids — microscopic raspberry‑like aggregates (sedimentary).
• “Suns”/“dollars” — flattened discs pressed along shale bedding.
• Botryoidal carpets — sparkling drapes of microcrystals on matrix.

Photo tip: Use side‑light ~30° and a white bounce card; striations will “scan” across cube faces and the brass warms without blowing out highlights.

Striated cube faces

At 10×, cube faces show fine parallel grooves (growth striations). On natural crystals these are uniform and straight; on fakes (cast/glued), they may look irregular or absent.

Framboidal textures

In sedimentary pieces, look for clusters of tiny spheres (framboids) building larger masses—like raspberries made of glitter.

Twin clues & inclusions

Occasional penetration twins produce X‑like intergrowths. Tiny magnetite or galena inclusions are common companions in vein material.

Gold

Soft, malleable (2.5–3 Mohs), yellow streak, high SG (~19). Gold smears when cut; pyrite crumbles on a sharp blow. Gold forms nuggets/veins, not striated cubes.

Chalcopyrite

Softer (3.5–4), deeper brassy‑gold with frequent iridescence; crystals are tetrahedra/disphenoids, not crisp cubes; streak tends black‑green. Will not scratch glass as readily as pyrite.

Marcasite (FeS₂)

Polymorph of pyrite but orthorhombic: spearhead/cockscomb radiating crystals; paler, more tin‑white tone; less stable in humid air. Lacks pyrite’s striated cubes.

Pyrrhotite

Bronze to coppery; often magnetic; softer (3.5–4.5). Rarely forms clean cubes; tarnishes quickly.

Bornite (“peacock ore”)

Dark, quickly iridescent purple/blue; much softer. The rainbow betrays it.

Quick checklist

• Striated cubes/pyritohedra → pyrite.
• Soft, malleable, yellow streak → gold.
• Magnetic bronze → pyrrhotite.
• Spearhead radiating clusters → marcasite.

Cubes from Spain

Navajún, La Rioja (Spain) produces legendary, razor‑edged cubes—often perched in clay host rock like geometric sculptures.

Brilliant clusters

Huanzalá Mine (Peru) is known for lustrous, intergrown pyrite clusters that gleam like polished metal. Italy’s island of Elba has classic crystals as well.

Pyrite “suns”

Flattened discs (“pyrite suns/dollars”) weather from Pennsylvanian‑age black shales—famously from the coal mines of Illinois—pressed perfectly along bedding planes.

Everywhere, really

From hydrothermal veins to shale beds, pyrite is global; chances are your local geologic map has some.

Keep it dry

• Display in low‑humidity conditions; add fresh silica‑gel packs to cases.
• Avoid long soaks; if rinsed, dry thoroughly (moving air helps).
• Skip bleach/acids; they speed tarnish and can etch surfaces.

Cleaning

• Use a soft brush or air bulb to lift dust from striations.
• Fingerprints can dull the glow; wipe gently with a dry, clean microfiber.
• For stubborn clay in crevices, a dry wooden skewer is safer than water.

Stability notes

• Some sedimentary pyrites (and marcasite) are humidity‑sensitive and may form sulfate crusts (“pyrite disease”).
• Isolate any specimen showing powdery bloom; reduce humidity and consult conservation tips before cleaning.
• Cabochons & beads: fine for jewelry with mindful wear; avoid harsh sweat/lotions + store dry.

Streak test (tiny & tidy)

On an unglazed porcelain streak plate, pyrite leaves a greenish‑black streak. (Use a corner on a tiny grain—no need to scuff a display face.)

Striation safari

With a 10× loupe, follow the growth lines across a cube face. They’re perfectly parallel—like a tiny record groove pressed by geology’s turntable.