Pyrite

What it actually is

Iron sulfide, FeS2, cubic, hardness 6 to 6.5, with no cleavage but conchoidal to uneven fracture.

Its crystal habits are the striking feature. Pyrite grows as cubes, as pyritohedra with twelve pentagonal faces, and as octahedra, and the cubes from Navajun in Spain are so sharp and so perfectly formed that first-time viewers routinely assume they are man-made.

The name comes from the Greek pyrites lithos, stone of fire, because it strikes sparks against steel. That use goes back to the Neolithic and continued into the era of wheellock firearms, which used pyrite before flint.

Pyrite is brittle and shatters under a pin, where gold is malleable and dents. Its specific gravity is about five where gold's is over nineteen, and its streak is greenish black where gold's is golden.

In humid conditions pyrite oxidises to iron sulfates and sulfuric acid, expanding, cracking and eventually crumbling. This is called pyrite decay or pyrite disease.

Where it comes from

Navajun in La Rioja, Spain, produces the famous perfect cubes in marl matrix, and they dominate the specimen market.

Peru produces spectacular crystal groups, Italy's Elba produced classic material, and China, Bulgaria and the United States all supply specimens.

Pyrite is one of the most widespread sulfide minerals and occurs in hydrothermal veins, in sedimentary rocks including coal, and as a fossil replacement, which is why some ammonites come out of the ground gold.

Industrially, pyrite was mined for sulfur and sulfuric acid production into the twentieth century, and the Rio Tinto district in Spain worked pyrite ores for millennia.

Fossil fuel geology matters here too: pyrite in coal is the main source of the sulfur that caused acid rain.

What the traditions claimed

Pyrite's history is practical before it is symbolic. Fire-striking is the oldest use, and pyrite nodules appear with flint in prehistoric fire kits.

The fool's gold association is the dominant cultural thread, and it is genuinely historical: prospectors during gold rushes repeatedly mistook it, and the phrase entered English as a general metaphor for deceptive appearance.

Inca and Mesoamerican cultures polished pyrite into mirrors, and pyrite mirror backs appear in Maya and Aztec contexts, which connects it loosely to the scrying tradition that obsidian dominates.

Roman and medieval sources are relatively quiet about it.

Modern practice reads pyrite as a stone of confidence, will and prosperity, largely on the strength of its gold appearance. That reading is twentieth-century and is worth treating carefully, since prosperity claims are exactly the kind this library does not endorse.

How it is worked with

The honest thing pyrite offers is the geometry. A Navajun cube on a desk is a genuinely startling object and a good demonstration that nature produces flat faces and right angles without any help.

The nerve reading, about acting with confidence in a situation where appearance matters, follows the fool's gold story and is more interesting than the prosperity one.

The care requirements are the real subject. Keep pyrite dry, keep it away from paper, cardboard and labels, and store it separately from other minerals. A specimen developing white powder, a sulfurous smell or fine cracks is decaying and should be isolated immediately.

Low humidity is the only reliable prevention. Silica gel in a sealed container helps.

Do not put pyrite in water, and never use it for gem waters, since it produces acid.

Buying it honestly

Pyrite is too cheap to fake, and the identification questions run the other way: people ask whether they have found gold.

Three tests settle it. Pyrite is brittle and shatters under a pin while gold dents. Pyrite's streak on unglazed porcelain is greenish black while gold's is golden yellow. And gold is about four times denser.

Marcasite in jewellery is almost always pyrite, since true marcasite is too unstable to set. That is a long-standing trade usage rather than a deception.

Some pyrite is stabilised or lacquered to slow decay, which is legitimate and should be disclosed.

The genuine buying risk is condition: a specimen already beginning to decay will continue to, and no treatment reliably stops it. Examine pieces for white efflorescence and hairline cracks before buying.