“Pyrite, like this from the Victoria Mine in Rioja, Spain, often forms in sharply defined cubes. You can understand why it’s called "fool’s gold." Universal History Archive/Universal Images Group via Getty Images
Martin Frobisherthought he’d hit the jackpot. The year 1576 found this English explorer and legal pirate — he was sanctioned by the crown to plunder enemy treasure ships — seeking the Northwest Passage, the undiscoveredArctic sea route that links the Atlantic and Pacific oceans.
He found something else instead — Labrador, Canada, and what is now Frobisher Bay. But weeks later he sailed west and reached icy Baffin Island, where he gathered a mineral sample that seemed to be flaked with gold. But it wasn’t. Not according to the Royal Assayer who identified the shiny bits as pyrite, also known as "fool’s gold."
Undeterred, Queen Elizabeth’s merchants sent Frobisher back to Baffin, where he gathered and shipped 1,400 tons (1,270 metric tons) of ore. Most of it was worthless; in a few tested samples, the gold content was only five to 14 parts per billion.
Though he longed to abandon Baffin and go exploring again, Frobisher spent years fruitlessly hunting Arctic treasure. And it was all because of that pyrite.
“A vein of gold is seen here in a hydrothermal sample of quartz taken from an unspecified mine at or near Grass Valley in northeast-central California. You can see how different the gold vein is from the pyrite above.James St. John/Flickr/(CC BY 2.0)
Elements and Compounds
Captain Christopher Newport could likely sympathize. As the leader of Jamestown, England’s first permanent settlement in North America, he was constantly getting tricked by New World "gold" that turned out to be — you guessed it — pyrite.
So let’s say you’re a prospector, or maybe just a bright-eyed field geologist. How do you avoid pyrite’s trickery?
Before we get into that, it might be a good idea to explain what pyrite actually is in the first place.
Real gold is a chemical element, a substance no ordinary chemical process — like electrolysis or heating — can break down. If you’ve got a classroom periodic table handy, look for gold between platinum and mercury.
Gold’s chemical symbol is "Au" (derived from the element’s Latin-language name, "aurum"). A fun way to remember this is to say to yourself, "A! U! Give me back my gold!" For maximum entertainment value, use a Brooklyn accent.
Pyrite is different. Unlike gold, it’s a compound made up of two different elements: iron and sulfur. That’s why it’s often referred to by the name "iron sulfide."
Scientists write out pyrite’s chemical formula as "FeS2." You see, iron and sulfur’s chemical symbols are, "Fe" and "S," respectively. And each pyrite molecule contains one iron atom along with two sulfur atoms.
Telling gold and pyrite apart really isn’t that difficult if you know what you’re doing. Ever watch the Olympics? Then you’ll probably know those world-class athletes love to bite their gold medals in front of the cameras. (Seriously, it happens a lot.)
The practice comes from the old belief that one can bite gold coins to see if they’re counterfeit. Supposedly, nibbling on any coin with a high gold content leaves bite marks behind.
The truth’s more complicated, though, but the custom has a basis in fact. On the Mohs’ scale, which rates the hardness of gems and minerals, gold has a ranking of 2.5 to 3. As elements go, it’s rather soft so a gold nugget can easily be scratched with a pocket knife.
Pyrite has the advantage here; it’s a bit harder and comes in at 6 to 6.5 on the Mohs’ scale. Forget knives; you’d need a high-quality metal file to scratch this stuff.
Steel hammers are another tool that can give the game away. Hit some pyrite with one of these beauties and it’ll send sparks flying. If you’re persistent enough, the pyrite will shatter and eventually get reduced to a powder.
None of that happens when you strike gold with a hammer: No sparks, no powder. Instead, you might just end up expanding or flattening the sample. Not only is gold soft, it’s malleable to boot.
See How I Glitter
Visually, both materials are yellowish, but gold is less brassy in hue. It also doesn’t form cube-shaped crystals, as pyrite often does. On the contrary, most of the gold encountered in the field takes the form of either flakes or lumpy nuggets.
Gold also will leave a yellow streak behind if it’s rubbed against a bit of porcelain or white ceramic tile. Repeat this same experiment with pyrite and it will leave a darker, greenish-black line.
If you’re still in doubt, the nose knows. Although gold is pretty much odorless, pyrite has a faint smell — and it smells like rotten eggs. (Again, it’s loaded with sulfur.)
But where things can get confusing is gold and pyrite sometimes turn up in the same deposits. Remember, Frobisher’s ore did contain some genuine gold — albeit a teeny, tiny amount.
If "real" gold keeps eluding you, don’t despair. Fool’s gold isn’t completely useless. Like we already mentioned, it can be used to produce sparks, and thereby start fires. That made pyrite a valuable commodity in ancient and prehistoric societies. Indeed, the word "pyrite" itself came from a Greek term for "firestone."
Tomorrow may bring a new appreciation for iron sulfide. In 2020, scientists at the University of Minnesota used electric voltage and an ionic solution to successfully turn pyrite into a magnetic material. This breakthrough could lead to low-cost, sulfur-based solar cells down the road — giving fool’s gold a bright future in the green energy industry.
NOW THAT’S INTERESTING
He never found the Northwest Passage, but Martin Frobisher was knighted in 1588 for fighting the Spanish Armada.