Pyrite vs. Gold — Key Properties
| Property | Pyrite (FeS₂) | Gold (Au) |
|---|---|---|
| Dana Class | Sulfide — Class 2 | Native Element — Class 1 |
| Crystal System | Isometric | Isometric |
| Crystal Habit | Cubic, pyritohedral, octahedral | Dendritic, arborescent, massive, wire |
| Mohs Hardness | 6 – 6.5 | 2.5 – 3 |
| Streak | Greenish-black to black | Golden yellow |
| Specific Gravity | 4.9 – 5.2 | 15.6 – 19.3 |
| Fracture | Conchoidal, brittle | Hackly, malleable |
| Luster | Metallic, splendent | Metallic |
| Odor when struck | Sulfurous | None |
During the California Gold Rush, newly arrived miners encountered pyrite so often and mistook it so reliably that "fool's gold" entered the language as shorthand for a valuable-looking thing that turns out to be worthless. The term outlasted the Gold Rush by about 170 years and is now used to describe bad investments, deceptive partnerships, and strategic mistakes.
That reputation is only partly fair to pyrite. It is, mineralogically speaking, a remarkable mineral in its own right forming perfect cubes and pyritohedra in ways gold simply cannot. But the visual similarity is real, and it has fooled people for millennia. Here is how to tell the two apart.
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What makes them look alike
Both pyrite and native gold have a bright metallic luster and a warm yellow color. In poor light, in a stream, or at first glance in a hand specimen, the resemblance is genuine. Both are isometric — they crystallize in the same overall crystal system, which means they can produce superficially similar-looking blocky or granular masses.
The color of pyrite comes from its electronic band structure: the Fe–S bonding creates a material that strongly reflects yellow-wavelength light. Native gold's yellow color comes from relativistic contraction of the 6s electron orbital — a stranger and more interesting explanation, but the visual result is similar.
Pyrite forms perfect cubes. Gold does not. If the specimen has sharply geometric crystal faces with striated surfaces, you almost certainly have pyrite.
How to tell them apart in the field
There are several reliable tests you can perform without laboratory equipment. They work because pyrite and gold have fundamentally different mineralogical identities — the visual similarity is entirely superficial.
1. Check the crystal form
Pyrite commonly forms sharp cubes, pyritohedra (12-faced forms with pentagonal faces), and octahedra — often with striated crystal faces. Gold rarely forms well-defined crystals; it typically occurs as irregular masses, dendritic growths, wire forms, flakes, or nuggets. Clear geometric faces with parallel striations means pyrite.
2. Test the hardness
Pyrite has a Mohs hardness of 6–6.5 and will scratch glass easily. Gold has a hardness of 2.5–3 — softer than a copper coin. Scratch the specimen against an unglazed porcelain streak plate. Gold leaves a golden mark and gets scratched itself; pyrite scratches the surface without leaving a trace of itself behind.
3. Run a streak test
Drag the specimen across an unglazed porcelain tile. Pyrite produces a greenish-black to black streak. Gold produces a golden yellow streak identical to its surface color. One of the most reliable single tests available without any equipment.
4. Test malleability
Gold is one of the most malleable metals in existence — you can deform it with a thumbnail. Pyrite is brittle and fractures with conchoidal breakage rather than deforming. Press a steel blade against the specimen. Gold dents. Pyrite chips or shatters.
5. Feel the weight
Gold has a specific gravity of 15.6–19.3 — among the highest of any naturally occurring element. Pyrite's is 4.9–5.2. A piece of true gold feels dramatically, surprisingly heavy for its size. Even a small nugget feels wrong in the hand — unexpectedly substantial.
6. Strike it and smell it
Striking pyrite against a hard surface releases a faint sulfurous odor from liberated sulfur. This is diagnostic. Gold produces no odor under any field conditions.
Why pyrite still matters
Pyrite's reputation as worthless is geologically misleading. In the field, pyrite is frequently a pathfinder mineral for actual gold deposits — both form in the same hydrothermal systems, and disseminated gold in sulfide-rich ore zones sometimes occurs alongside pyrite, or even within pyrite grains as microscopic inclusions. Experienced economic geologists treat pyrite as a data point, not a disappointment.
Classification note (Dana's System): Pyrite is classified under Sulfide Class 2, space group Pa3̄. Gold falls under Native Elements Class 1, space group Fm3̄m. Both are isometric, but their atomic arrangements, bond types, and physical properties are fundamentally different.
Frequently Asked Questions
Can pyrite and gold occur in the same rock?
Yes. Both form in hydrothermal ore systems, and significant gold deposits have been found in sulfide-rich zones where pyrite is abundant. In some Carlin-type deposits, microscopic gold is hosted within pyrite grains — making pyrite a useful indicator mineral rather than a false positive to dismiss.
Why does pyrite produce a black streak if it looks golden?
A mineral's streak — the color of its powder — is often different from its surface color. The yellow surface color of pyrite comes from how its crystal structure reflects light in bulk. When powdered on a streak plate, that optical effect disappears and the intrinsic color of the iron sulfide powder is revealed: greenish-black to black.
Is chalcopyrite also sometimes mistaken for gold?
Yes. Chalcopyrite (CuFeS₂) has a more brassy, iridescent yellow that is often more convincingly gold-colored than pyrite. The same identification tests apply — streak test (greenish-black), hardness (3.5–4), and its characteristic iridescent tarnish. Chalcopyrite is the primary ore of copper.
What is the pyritohedron and why is it named after pyrite?
The pyritohedron is a 12-faced crystal form (a pentagonal dodecahedron) so characteristic of pyrite that it was named after the mineral. Each face is a pentagon, and the form belongs to the pyritohedral symmetry class (2/m3̄) — a lower-symmetry subgroup of the full isometric class. It is diagnostic for pyrite and rarely seen in gold.
Sources & Further Reading
- Gaines et al. — Dana's New Mineralogy, 8th edition (1997) — Pyrite and Gold entries
- Mindat.org — Pyrite mineral data
- Mindat.org — Gold mineral data
- Handbook of Mineralogy — Pyrite data sheet
- USGS — Gold — Mineral Resources Program