Short answer: pyrite is iron sulfide (FeS2), a brassy, metallic mineral nicknamed fool's gold. It often grows as near-perfect cubes with striated faces. Unlike gold, it is hard and brittle and leaves a greenish-black streak. Its name comes from the Greek for fire, because it throws sparks when struck. Kept dry, it lasts; left damp, it can slowly decay.
What is pyrite?
Pyrite, or iron pyrite, is iron disulfide, FeS2. It is the most abundant sulfide mineral on Earth. It forms in quartz veins, in sedimentary and metamorphic rocks, in coal beds and as a replacement mineral in fossils, where it can turn an ammonite or a shell into a golden copy of itself.
It is brass yellow with a bright metallic luster, rates 6 to 6.5 on the Mohs scale, and is heavy: with a specific gravity of about 5, a piece of pyrite weighs nearly twice as much as a piece of quartz the same size.
Why does pyrite form perfect cubes?
Pyrite crystallizes in the cubic, or isometric, system: its atoms are arranged with the symmetry of a cube. When pyrite grows freely, the outside of the crystal follows that inside order, and the result can be startlingly geometric. Cubes from Navajún in La Rioja, Spain, sit in their soft gray host rock looking machined.
Cubes are not the only shape. Pyrite also grows as octahedra and as pyritohedra, a twelve-sided shape with five-sided faces named after the mineral itself. Fine parallel grooves, called striations, often run across the faces of pyrite cubes, alternating direction from one face to the next. Pyrite also forms flat discs, clusters, nodules and sparkling masses.
Why is it called pyrite, and why fool's gold?
The name comes from the Greek pyr, "fire". Strike pyrite against steel or another hard stone and it throws sparks, which made it useful for starting fires. It was even used to make the sparks in early flintlock firearms.
The nickname fool's gold comes from prospectors who mistook its golden shine for the real thing. The joke has a twist: pyrite and gold form under similar conditions, and some pyrite contains small amounts of real gold. Pyrite is mined as an ore of gold in some deposits.
How do you tell pyrite from gold?
| Pyrite | Gold | |
|---|---|---|
| Color | Brassy, pale yellow; tarnishes darker | Rich, buttery yellow; does not tarnish |
| Shape | Often cubes and other crystals with flat faces | Usually grains, flakes and nuggets |
| Hardness | Hard and brittle; shatters when hit | Soft and malleable; dents and flattens |
| Streak | Greenish-black to brownish-black | Golden yellow |
The streak test is the quickest: rub the specimen across unglazed white porcelain. Pyrite leaves a dark greenish-black line. Gold leaves a golden one.
Marcasite is the mineral most similar to pyrite. It has the same chemistry but a different crystal structure and a paler brass color. In older jewelry, "marcasite" usually means small faceted pieces of pyrite set in silver, a style popular in the Victorian era.
Where does pyrite come from?
Pyrite is found worldwide. Collectors know it especially from Navajún, Spain, for its cubes, and from Peru, including the Huanzala mine, for lustrous intergrown crystal clusters. It also occurs in coal mines, in metal mining districts and in fossil beds.
What is pyrite traditionally associated with?
In modern crystal traditions, pyrite is associated with abundance, prosperity and good fortune, a natural association for a stone that looks like gold. It is also seen as a protective stone and a symbol of confidence and willpower, and it is commonly linked with the solar plexus chakra. People keep pyrite cubes and clusters on desks and near entrances for these reasons.
These are traditional associations and beliefs, not properties you can measure in the mineral.
Raw, tumbled and polished pyrite
Natural cubes and clusters are the most prized pyrite pieces, valued for sharp faces and bright luster. Polished pyrite and tumbled stones have a mirror-like metallic shine. Pyrite also appears as sparkle in other stones, such as the golden flecks in lapis lazuli.
Pyrite in fossils
Pyrite often forms where organic material decays without oxygen, in muds and shales. As a result it can replace the shell or tissue of a buried organism, producing golden fossils, such as ammonites whose shells have turned to pyrite. These pieces are striking, but fine-grained pyrite is the kind most prone to decay, so pyritized fossils need especially dry storage.
How do you choose a pyrite specimen?
- Sharp crystal faces and crisp edges on cubes and clusters.
- Bright, untarnished luster. Some tarnish is natural, but a dull, powdery surface may be a sign of decay.
- No cracks spreading through the piece, and no powder in the box.
- Matrix: cubes still set in their host rock, like those from Navajún, are prized.
Pyrite jewelry
Pyrite is used in beads and in marcasite-style jewelry. It is hard enough to resist scratches, but it is brittle and can be damaged by moisture and sweat over time, so it suits pieces that are kept dry and not worn in water.
How do you care for pyrite?
Pyrite's one weakness is moisture.
- Keep it dry. In contact with air and water, pyrite slowly breaks down into iron oxides and sulfuric acid. Signs include a powdery coating, cracks that spread, crumbling, and acid damage to nearby labels and boxes. Fine-grained and finely crystalline pyrite decays faster than large, solid crystals.
- Do not wash or soak it. Wipe it with a dry, soft cloth or brush.
- Store it in a dry place, away from bathrooms and damp basements, and out of humid air.
- Keep it apart from other specimens, since decaying pyrite releases acid that can harm what it touches.
Sound, well-crystallized pyrite kept dry can stay bright for generations.
How can you tell real pyrite?
- Weight: pyrite is heavy for its size. Painted or plated imitations feel light.
- Streak: a greenish-black streak on unglazed porcelain.
- Hardness: pyrite scratches glass. Gold and most metallic paints do not.
- Crystal faces: natural cubes show striations on their faces.
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Frequently asked questions
Can pyrite get wet?
It should be kept dry. Water and humid air slowly break pyrite down into iron oxides and sulfuric acid. Wipe it with a dry cloth instead of washing it.
Is pyrite natural if it forms perfect cubes?
Yes. Pyrite's atoms are arranged with the symmetry of a cube, so natural crystals often grow as remarkably sharp cubes. No cutting is needed.
Does pyrite contain gold?
Sometimes. Pyrite and gold form under similar conditions, and some pyrite contains small amounts of gold. Pyrite is mined as an ore of gold in some deposits, though a typical specimen holds only a trace, if any.
What is pyrite traditionally associated with?
In crystal traditions it is associated with abundance, prosperity, protection and confidence, and with the solar plexus chakra. These are traditional meanings rather than measurable properties.
Why is my pyrite turning white?
A powdery coating is a sign of decay caused by moisture: the pyrite is breaking down into iron oxides and sulfuric acid. Move it to a dry place, brush off the powder, and keep it away from other specimens. Fine-grained pyrite is the most vulnerable.
Sources
- Wikipedia, Pyrite: https://en.wikipedia.org/wiki/Pyrite
- Geology.com, Pyrite: https://geology.com/minerals/pyrite.shtml
- Wikipedia, Pyritohedron: https://en.wikipedia.org/wiki/Pyritohedron
- Wikipedia, Marcasite: https://en.wikipedia.org/wiki/Marcasite