Pyrite
FeS₂ · Cubic · also called fool's gold, iron pyrite
Pyrite is the mineral behind essentially every 'I think I found gold' photograph. It is brassy, metallic, and common in rocks worldwide. It is also six times harder than gold, brittle rather than malleable, and leaves a dark green-black streak. Its habit of growing into perfect striated cubes makes it a genuinely attractive specimen mineral in its own right.
Properties
- Mohs hardness
- 6–6.5
- Streak
- green black, black
- Lustre
- metallic
- Colour
- Pale brass-yellow with a faint greenish cast. Fresh surfaces are bright; older ones dull to brown as the sulfide oxidises.
- Cleavage
- Cleavage is poor and rarely visible on hand specimens.
- Fracture
- Conchoidal to uneven, and distinctly brittle
- Specific gravity
- 4.9–5.2 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm pyrite
- Brittle: a knife point crushes it to dark powder rather than cutting a groove.
- Streak on unglazed porcelain is greenish black — the single fastest way to rule out gold.
- Frequently forms sharp cubes with fine parallel striations on the faces. Gold never does this.
What pyrite gets confused with
Each of these is settled by one test. Run that test first.
Scratch it. Gold smears and dents; pyrite crumbles. Streak colour confirms it: golden versus greenish black.
Chalcopyrite is softer (3.5–4) and can be scratched by a steel nail. Pyrite cannot.
Marcasite is paler, almost tin-white on fresh surfaces, and forms blades and cockscombs rather than cubes.
Where it is found
Almost everywhere — hydrothermal veins, coal seams, shale, and slate. Cubes weather out of soft shale and turn up loose in creek beds and roadcuts.
Is it worth anything?
Common pyrite has little cash value. Sharp, undamaged crystal groups from known localities (Navajún in Spain, for example) do sell well as display pieces. Note that pyrite in humid storage slowly breaks down and can damage nearby specimens.