Stefan Chin: In January 1848, a carpenter in California found some flakes of gold in the waterway supplying the sawmill where he was working. It was a lucky find, and before long, thousands of other hopeful prospectors and thieves descended on the area to find their own golden treasure. Thus began the Californian Gold Rush, which saw more than 300 tonnes of gold extracted from the area around the Sacramento River. While seeking their fortunes, prospecting hopefuls needed to be careful they weren’t tricked.

Fool’s gold, a shiny yellow metallic mineral called pyrite, was everywhere. And it wasn’t worth a dime, especially compared to the gold in them Thar hills.. But as it turns out, there was another kind of fool’s gold out there called Chalcopyrite, meaning there were twice as many tricks to watch out for.

These were the days before fancy chemical tests, they needed a rough and ready field test to tell them they’d gotten the good stuff. And it turns out that this is the  simplest and most effective test.

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[1:02] Stefan: The California gold rush ultimately brought about a quarter of a million hopefuls to the area, hoping to strike it rich. But gold fever wasn’t limited to America. Around the same time, Australia  experienced its own gold rush, and all through the second half of the 19th Century, gold hunters flocked to  locations in Canada, South Africa and elsewhere in America at  the first whiff of a gold find. Prospectors had to be reactive, because it was hard to predict where the gold would be.

Gold can be found in lots of different settings, can exist as tiny flakes or giant nuggets, and might show up in streams and lakes or still be embedded in the rock itself. It’s a moving target. Part of the problem is how gold  gets to the places we find it.

See, gold is first deposited in hydrothermal veins that cut through existing rocks. But it tends to be in small quantities and really spread out, so it’s easy to miss. But when those veins get  weathered and the gold washes out, it gets redeposited in river and lake sediments called placer deposits.

And here, among modern and ancient sediments, is where you can find amounts of gold worth your time. The thing is, the geological processes that help to concentrate gold into valuable deposits also tend to gather together other minerals, like pyrite and its cousin Chalcopyrite. Chalcopyrite is the fool’s  gold you’ve never heard of.

It’s similar to pyrite, which has  isometric crystals made of iron and sulfur, but it has extra copper in it, which makes these Tetragonal crystals that like to grow together into one another. When Chalcopyrite reacts with oxygen in the air, it can also turn a bunch of psychedelic colors, from green to peacock purple. But in its natural un-oxidized state, it’s a brassy gold.

Not only is it gold in color, but it’s pretty soft as far as minerals go. You can scratch it with a copper coin, which means it can look about as soft as real gold to the untrained eye. Both pyrite and Chalcopyrite occur alongside gold in old hydrothermal systems, where metal rich fluids have flowed through existing rock and left behind their mineral treasure.

It’s little wonder that these two fool’s golds have wound up confusing optimistic prospectors. Like Jaques Cartier. Cartier - no relation to the French jewelry guy - was exploring the St.

Lawrence River in Canada, in the 1540s, when he found what he thought to be gold and diamonds. It was only when he got his treasures home to France that he discovered that it was just a bunch of quartz and pyrite. So it goes to show that real people have been actually fooled by these lookalike minerals.

Huge Bummer, dude. And even during the later gold  rushes of the 19th Century, prospectors would have needed  quick ways of distinguishing trash from treasure in the field. So, in lieu of any fancy geochemical analysis, rock hunters could have used a streak test, to tell all of their shiny golden minerals apart.

All you need to do is treat your rock like a crayon. Just drag it along an un-glazed white ceramic surface, and look at the color of the streak left behind. And you might be surprised,  because a mineral’s streak isn’t always the same colour as the rest of the crystal.

That’s because the color of a crystal depends as much on geometric structure as it does the chemistry. Minerals like opal and Labradorite owe much of their color to the way light bends through their structure. And trace elements contaminants  can also throw off the color too.

For instance, calcite is a mineral that comes in lots of colors depending on what trace impurities are caught up in the crystal, but no matter the color of the whole piece, its streak will always be white. So doodling with your rock removes the  diffraction and contamination factors, meaning that the streak color will be the same no matter what. Of course, the streak test only works if your mineral is softer than the ceramic tile you're dragging it across, so about a 7 on the Mohs hardness scale.

If they’re not, then all you’d get is a scratch in your tile. Which, conveniently enough, applies to all kinds of gold - foolish or otherwise, are all lower than seven on the moh's hardness scale. Even though gold and its lookalikes all have soft-ish, shiny yellow crystals, they have different colored streaks.

Gold, fittingly, leaves a golden yellow streak on the plate. On the other hand, pyrite’s streak is a greenish, somewhat metallic black. And as for Chalcopyrite?

The streak is greenish black, just like pyrite. All it takes is a ceramic tile and a pair of eyes! And while Chalcopyrite might not have been what the gold rush prospectors wanted to find, it’s not exactly trash, either.

Today, it’s one of the main ores for all the copper we use in our pipes, wires, and construction. It’s also used to make sulfuric acid, which has a role in fertilisers  and industrial chemistry. And places like Copperfields Mine in Ontario and Olympic Dam in Australia are now better known for their copper-bearing Chalcopyrite than for the gold that’s found alongside them!

Who’s the fool now? What’s even better, Rocks Box subscribers get a piece of this beautiful golden mineral with its green-black streak, right to their door. Every month we pick out a special mineral or fossil with a story to tell and send them your way.

If you wanna get on the wait-list or peruse our shop from some of our greatest hits of the past, head over to SciShow.rocks or click the link in the description. Thanks for watching!

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