What is fool's gold?
Fool's gold is usually pyrite, hardness 6 to 6.5, density 5.018. Gold is hardness 2.5 to 3 and density 19.3, so it is the softest and heaviest of the four.
Fool's gold is usually pyrite, FeS2, hardness 6 to 6.5, density 5.018, a brass-yellow sulphide mineral that streaks greenish to brownish black and forms cubic crystals to 25 cm. Gold is hardness 2.5 to 3 and density 19.3. A 2 cm cube of gold weighs 154 g; the same cube of pyrite weighs 40 g.
In short
- Fool's gold is not one mineral. Four things get called it: pyrite, its dimorph marcasite, chalcopyrite, and weathered flakes of mica, which split on a perfect cleavage. Each is separated by a different test, so 'is it pyrite' is the wrong first question.
- Gold is soft. This surprises everyone. Gold is hardness 2.5 to 3 — a copper coin will mark it — and pyrite is 6 to 6.5, hard enough to scratch glass. The hardness test runs the opposite way to most people's intuition.
- Malleability is the decisive test and costs nothing. Press a grain with a steel pin on a hard surface. Gold flattens and stays flattened. Pyrite, marcasite and chalcopyrite shatter into brittle fragments.
- The streak plate settles it in one drag. Gold leaves a gold-yellow streak. Pyrite leaves greenish black to brownish black. Chalcopyrite leaves greenish black. The streak is of the powder, and the powder does not lie about colour.
- Density is the arithmetic that ends the argument. 19.3 against 5.018 is a factor of 3.8. A gold nugget feels wrong in the hand in a way nothing else does, and a specimen that feels ordinary for its size is not gold.
| Mineral | Formula | Hardness | Density | Streak | The test that settles it |
|---|---|---|---|---|---|
| Gold | Au | 2.5 to 3 | 19.3 | Gold-yellow | Very malleable and ductile — it flattens under a pin instead of shattering |
| Pyrite | FeS2 | 6 to 6.5 | 5.018 | Greenish black to brownish black | Hard enough to scratch glass; cubic and pyritohedral crystals, striated |
| Marcasite | FeS2 | 6 to 6.5 | 4.887 | Greenish to brownish black | Same chemistry as pyrite, different structure. Hardness and streak do not separate them |
| Chalcopyrite | CuFeS2 | 3.5 to 4 | 4.1 to 4.3 | Greenish black | Softer than pyrite, brassier and yellower, tarnishes iridescent; tetrahedral crystals |
| Mica (muscovite, biotite) | Sheet silicates | 2.5 along the cleavage | 2.77 to 3.3 | White | Splits into flakes that bend and spring back. Nothing metallic does that |
The density arithmetic, worked, so it ends the conversation
Density is the property that separates gold from everything it is confused with by a margin so large no measurement error can close it. Here is the calculation with real numbers.
Take a cube 2 cm on a side. Its volume is 2 x 2 x 2 = 8 cubic centimetres.
- Gold, density 19.3: 8 x 19.3 = 154 grams.
- Pyrite, density 5.018: 8 x 5.018 = 40 grams.
- Marcasite, density 4.887: 8 x 4.887 = 39 grams.
- Chalcopyrite, density 4.1 to 4.3: 8 x 4.2 = 34 grams.
- Muscovite, density 2.77 to 2.88: 8 x 2.8 = 22 grams.
Gold is 3.8 times heavier than pyrite for the same size, and seven times heavier than mica. Substitute your own dimensions — the multiplier does not change. This is why panning works at all, and it is why anyone who has actually held gold recognises the next piece instantly by weight alone.
To measure rather than estimate, weigh the specimen in air, then weigh it suspended in water; the density is the air weight divided by the difference. Our page on identifying a mineral specimen covers the method, and is my rock worth anything covers the question that usually sits underneath this one.
Hardness runs the wrong way, and that is the useful part
Ask someone to guess and they will say gold is the harder of the two, because it is the valuable one. It is the softest mineral in the table.
Gold is hardness 2.5 to 3. Chalcopyrite is 3.5 to 4. Pyrite and marcasite are 6 to 6.5. Mica is 2.5 along its cleavage and about 4 across it. So the ordering, softest first, is gold, mica, chalcopyrite, then the iron sulphides a long way behind.
That gives you two clean thresholds using things in your pocket. A copper coin is conventionally about 3.5: it marks gold, marks mica, is marginal on chalcopyrite, and does nothing at all to pyrite. A steel knife or a good needle is about 5.5: it marks everything in the table except pyrite and marcasite, and it will not scratch window glass, while pyrite will.
So the single sentence to carry away is this: if it scratches glass, it is not gold. That disposes of the great majority of cases in about four seconds, and it works in the field with no equipment. The technique — and the several ways a hardness test gives a wrong answer — is covered on testing mineral hardness properly.
Malleability: the one test that is actually diagnostic
Hardness and density both narrow the field. Only one property in this group belongs to gold and to nothing else it is confused with, and that is malleability.
The Handbook of Mineralogy gives gold's tenacity as very malleable and ductile, with a hackly fracture. Pyrite, marcasite and chalcopyrite are all recorded as brittle. Mica is flexible and elastic, which is a third and different behaviour.
The test: put the grain on a hard flat surface and press it firmly with a steel point, or tap it once with a small hammer. Gold spreads. It deforms into a flake and stays deformed, and the flake has bright torn edges. Pyrite and chalcopyrite crumble to angular fragments and a dark powder. Mica flattens, then springs back when released — elastic rather than malleable, which is a distinction you can feel through the pin.
The caveat is real: this test is destructive to the grain you use it on, so use a loose fragment and never a crystal you would want to keep. Pyrite forms superb cubes and pyritohedra to 25 cm and they are worth far more intact than as evidence. If the material is crystallised and sharp, look at the form instead — cubes with striations running in different directions on adjacent faces are pyrite, and gold does not do that.
Which of the four you probably have, by where you found it
Setting narrows the answer before any test does.
In a stream bed or a pan, bright flakes that stay bright when wet and are heavy in the pan tail are most likely gold; flakes that flash in one orientation and go dark in another, and that ride away in the current, are mica. This is the commonest single mistake and it is resolved by a pin.
In shale, coal or a limestone nodule, brass-yellow material is most likely pyrite or marcasite, and there is no bench test that separates the two — the hardnesses are identical at 6 to 6.5 and the densities are 5.018 against 4.887, a difference of 2.6 per cent. Sedimentary settings are where decay is actually reported, which is the subject of why pyrite crumbles.
In a copper mine dump, or in ore with green and blue secondaries, brassier and yellower material at hardness 3.5 to 4 is chalcopyrite. It tarnishes iridescent and is the parent of almost everything on a Cornish or Namibian oxidised specimen.
In a hydrothermal quartz vein, both gold and pyrite are plausible and the tests above are the way to settle it. We will not put a value on any of this, here or anywhere on the site, because a number stated without seeing the material is a misleading number — and with gold in particular the assay, not the appearance, is what would matter. If what you are after is a good crystallised pyrite or a British gold specimen for a collection, that is a specimen brief, and the place to state it is a wanted list.