Gem library
Pyrite
Fool's gold, a cube of golden steel
Pyrite is an iron sulphide whose cubes are so geometric they look machined. Its metallic golden lustre earned it the nickname fool's gold — and its Greek name recalls that it has been used to make fire since prehistory.
- Hardness (Mohs)
- 6 – 6.5
- Density
- 4.95 – 5.10
- Crystal system
- Cubic
- Refractive index
- Opaque (metallic)
- Cleavage
- Indistinct
Mineralogical identity
- Family
- Sulphides
- Species
- Pyrite
- Variety
- Marcasite (polymorph), framboidal pyrite
- Chemical formula
- FeS₂
- Crystal system
- Cubic
- Classification
- Collector's mineral
- Rarity
- Very common
- Indicative price
- €0.10 – 3/ct
Etymology
From the Greek pyritēs lithos, «the stone that makes fire»: struck against steel it throws sparks — it served as a fire striker for millennia.
Physical & optical properties
- Hardness (Mohs)
- 6 – 6.5
- Density
- 4.95 – 5.10
- Refractive index
- Opaque (metallic)
- Cleavage
- Indistinct
- Fracture
- Conchoidal to splintery
- Lustre
- Brilliant metallic
- Diaphaneity
- Opaque
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- brass yellow, golden, iridescent grey
Formation, geology & history
The perfect natural cube
The pyrites of Navajún in Spain produce cubes with flat faces and sharp edges of astonishing crispness, often isolated in soft marl. Elsewhere pyrite also takes the form of pyritohedra (dodecahedra with pentagonal faces), octahedra, granular masses, radiating nodules (the «pyrite suns» of Illinois) and microscopic framboids whose formation sometimes involves bacteria. The cube faces often carry fine striations running perpendicular from one face to the next — a diagnostic marker. Its density of 5, very high, makes it markedly heavy in the hand: that is the first thing you notice.
Fool's gold: how not to be fooled
Confusion with gold has cost more than one prospector dearly, and Martin Frobisher shipped tonnes of golden ore back to England in 1578 that was nothing but pyrite. Yet the differences are clear: pyrite is brittle and crushes to black powder, gold is malleable and flattens; pyrite scratches glass (hardness 6), gold is soft (2.5); pyrite leaves a greenish-black streak on porcelain, gold a bright yellow one; and gold is two to four times denser again. Pyrite leans towards greenish brass yellow, gold towards a warm yellow.
Pyrite decay
In the presence of moisture and oxygen, pyrite oxidises to iron sulphate and releases sulphuric acid: the specimen becomes covered with a whitish or rusty efflorescence, swells, crumbles and can destroy labels, boxes and neighbouring minerals. It is the nightmare of museum conservators, known as pyrite decay. Collection specimens must be kept at low relative humidity (below 40 %), away from other sensitive pieces. Framboidal pyrites and pyrite on black shale are the most vulnerable.
How to recognise it
- Weight and metallic lustreDensity 5: very heavy in the hand. The lustre is frankly metallic, never vitreous. No translucency at all, even at the edges.
- Scratches glassHardness 6-6.5: unlike gold (2.5) and golden mica (2-3), it scratches glass without deforming.
- Black streak on porcelainRubbed on an unglazed porcelain biscuit it leaves a greenish-black streak. Gold leaves a golden one.
Imitations, treatments & confusions
- Confusion with native goldThe decisive test is malleability: crush a grain with a knife tip. Gold flattens into a flake, pyrite pulverises.
- Golden mica (biotite, phlogopite)Very light, scratched by a fingernail, splits into flexible sheets. Often mistaken for gold in rivers.
- MarcasiteSame formula as pyrite but orthorhombic: paler, more unstable, it degrades even faster. The word «marcasite» in antique jewellery in fact denotes cut pyrite.
Deposits & origins
Spain (Navajún, La Rioja — the most perfect cubes in the world), Peru (Huanzala: spectacular pyrites), Italy (Elba), China, Mexico, United States (Illinois), France (numerous deposits)
Care & precautions
- Never water: it triggers oxidation and pyrite decay, which is irreversible.
- Keep away from moisture, in a dry place, ideally with a desiccant in a closed case.
- Dust with a dry brush. Never use acidic products or household cleaners.
- Do not store it touching other minerals: the sulphuric acid released by a decaying pyrite attacks neighbouring pieces.
Traditions & crystal healing
- Chakras
- Solar plexus · root
- Zodiac signs
- Leo · Aries
- Element
- Fire · Earth
A stone of will and confidence in tradition, linked to fire and action: it is said to support determination and self-esteem. The Incas polished pyrite discs into mirrors, and alchemists saw in it gold in the making, an intermediate stage in the maturing of metals.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
How do I tell pyrite from gold?
Crush a grain: gold flattens (malleable), pyrite pulverises (brittle). Pyrite also scratches glass and leaves a black streak on porcelain; gold leaves a golden streak.
My pyrite is crumbling, what should I do?
That is pyrite decay, an oxidation linked to humidity. You can slow it by drying the specimen and keeping it below 40 % relative humidity, but you cannot reverse it.
Is pyrite toxic?
Not to the touch. But it is a sulphide: avoid inhaling its dust when sawing, and do not leave it soaking in water, because oxidation produces sulphuric acid.