Pyrite vs gold and how to tell fool's gold from real gold

These checks can only rule gold out. Colour won't settle it: other metals mixed with gold can shift its colour from silver-white to green or orange-red, according to the USGS gold booklet. And proven fool's gold can still hold gold you can't see.

Gold, pyrite and chalcopyrite compared

Gold's figures come from the USGS fool's gold answer and the USGS gold booklet. Its hardness and colour range come from James Madison University's native gold page. The other columns come from the University of Minnesota's pages on pyrite and chalcopyrite.

Home checks, least damaging first

Start with the checks that leave no mark, because looking and weighing cost nothing. The pin does most of the work. A streak rubs off part of the find, so leave it until last.

Look at the crystal shape

Fine parallel lines (striations) on some faces point to pyrite, which often occurs as small cubes to octahedrons that may show them. A 1942 paper in American Mineralogist says gold crystals with striated cube faces "have not been reported". A sharp, flat-faced cube is almost always pyrite: the same paper calls well-formed gold crystals extremely rare and simple gold cubes very rare. Gold can still grow as octahedra, dodecahedra and cubes. Press a pin anyway.

Gold also turns up as grains, scales, lumps, plates, leaves and wires. A shapeless lump of pyrite or chalcopyrite tells you nothing from its shape. Chalcopyrite usually forms masses without flat planes. A rusty brown crust is worth noticing too: pyrite weathers through iron sulphate minerals to limonite, and a rusty limonite layer may mean pyrite underneath.

Feel the weight in your hand or a pan

Gold's specific gravity is 16 to 19.3 and the waste rock around it is about 2.5, according to the USGS gold booklet. Pyrite's is 5 or above, according to the University of Minnesota. So a gold-coloured lump that feels no heavier than a stone its size isn't solid gold. A lump that feels heavy can still be pyrite, which the University of Minnesota says "feels heavier than average, even for a metallic mineral".

A gold pan works on the same difference in density. The USGS booklet says prospectors look for gold where "black sands", mostly magnetite, "have concentrated and settled with the gold". Whatever stays at the bottom of the pan still needs the pin test. Weight won't help with a tiny grain.

Press a pin into it

Press the point of a pin or needle into the gold-coloured part, ideally under a hand lens, so you can see whether the point dents the surface or chips it. The USGS says gold will "gouge or indent like soft lead", while pyrite, chalcopyrite and mica "flake, powder, or crumble". The reason is that gold is "the most malleable and ductile of metals" according to the USGS gold booklet. Pyrite and chalcopyrite are brittle.

Biotite mica "is not particularly yellowish", but its small flakes in stream sediment flash brightly enough to fool people panning.

If the pin dents it, stop and get it identified (see below). A grain too small to press a pin into can't be settled with these checks, and neither service below says it can identify one that small from a photo. We'd hold off sending anything until you have a piece big enough to test.

Streak only what crumbled

A streak tells you whether a crumbly find is mica, which streaks white, or a sulphide, which streaks dark. The mineral identification chart shows how to make a streak.

Telling pyrite from chalcopyrite

Streak won't separate the two. The University of Minnesota says hardness is "often the easiest way" to tell them apart: a nail or knife blade scratches chalcopyrite, but most knife blades leave pyrite unmarked, and pyrite will scratch glass. The chart linked above shows how to run these scratch tests. Chalcopyrite also tarnishes more iridescently and seldom forms good crystals.

Gold you can't see in rock

The USGS says pyrite "can have actual gold in its mineral structure". A USGS prospecting pamphlet printed in 1991 says lode gold "may not be visible in the rock". Its advice is aimed at prospectors sampling outcrops, mine workings and mine dumps, who usually send 3 to 5 pounds (about 1.4 to 2.3 kg) of "representative mineralized rock" to a commercial analytical laboratory or assay office.

Pyrite in a vein can be a pointer. The University of Minnesota says vein pyrite may reveal valuable minerals such as chalcopyrite and gold.

The odds sit heavily with pyrite. Pyrite is "very widespread" across many rock types, the University of Minnesota says, while the USGS gold booklet calls gold "relatively scarce in the earth". The British Geological Survey names only a handful of UK gold deposits found by commercial exploration since the mid-1980s, all in the Scottish Highlands and Northern Ireland.

A single garden or beach pebble is neither 3 to 5 pounds nor a representative sample of mineralised rock. If its gold-coloured parts crumbled under the pin, we wouldn't pay to have it assayed. An assay only fits the case the USGS pamphlet describes: pyrite in a vein or outcrop where you can collect pounds of rock that represent the whole.

Where to get a find identified

In the UK, the Natural History Museum's Identification and Advisory Service answers queries about natural history specimens found in the UK: email photos and a description to its earth sciences address, esid@nhm.ac.uk. The museum asks for a detailed description, where and when you found it, and its size, with a coin in the photo for scale.

In the US, check first whether your own state's geological survey identifies finds. The Association of American State Geologists lists the state surveys, with contact details. The New Mexico Bureau of Geology and Mineral Resources is one that does, through the State Mineralogist. Start by sending in-focus, well-lit photos, then by appointment bring the find to Socorro or post it with return postage. The Bureau's FAQ doesn't say whether it takes finds from outside New Mexico. Neither the Natural History Museum nor the Bureau says it values finds.