When last we spoke on the subject of fool's gold, I introduced you to several famous people who'd mistaken it for real gold. Imagine how they'd feel if we yanked them out of their respective eras and showed them you actually can get gold out of pyrite. It's just that you need techniques that weren't developed until generations after they were dead. And the gold in it isn't exactly visible to the naked eye.

Image shows a chunk of blue-gray rock speckled with patches of pyrite tarnished to a coppery red color. One flat square of pyrite gleams a bright gold.

A lovely cubic bit of pyrite in blueschist, which I found in a rock wall in Bothell, WA. Alas, no visible gold - but a splendid golden color! The cube is about a half-inch across.

Even before we learned how to extract gold from it, we humans found pyrite pretty useful. It got its name because you can use it to create excellent sparks and even use it to start fires, which was nifty in the days before flint and steel. This sparking ability also turned out to be a pretty spiffy feature when wheellock firearms were invented. You can heap pyrite up and leach it, creating copperas, which folks use for everything from nutritional supplements to ink to dye and more. It's a good source of sulfur and has been mined for making sulfuric acid for a long time. And it's got many more direct uses, some of which may surprise you. So it's actually quite useful for many other things, not just fooling people into thinking it's gold. But it's one of the most abundant minerals on earth, occurring in every major rock group everywhere on the planet. So mining a bunch of it and shipping it across an ocean wasn't a bit economical.

We've also learned that pyrite's good for telling us where to pay attention in our search for potential riches. If it's present in veins, we know it's a good idea to look around the area for other, more valuable minerals and metals - yes, even gold. We often associate quartz veins with gold, but there can be more pyrite than quartz in some gold deposits. We now have the ability to analyze it and its inclusions and use it to figure out where to look for various valuable stuff in a mine.

Now, here's why I didn't run this on April Fool's Day. If I'd handed you this phenomenally beautiful chunk of quartz and arsenopyrite, and said, "Here! Have some invisible gold!" you would've been like, "Ha ha, Dana, yeah, I know what day it is and I am not a fool!"

I'll let Rob describe it for you: "An AMAZING cluster of quartz crystals out of the collection of Ed David! These gemmy crystals are stacked upon one another as if by hand, with both terminations complete, sticking out on either side! The terminations on one side of the crystals are sprinkled with little yellow-tan muscovites. On the other side is a cluster of arsenopyrite (the quartz/arsenopyrite association is well-known from this locality). A show-stopper of a quartz specimen!"

A gorgeous hand sample of exquisite quartz crystals and shiny arsenopyrite. Photo courtesy Rob Lavinsky / iRocks.com (CC-BY-SA-3.0).

And you wouldn't have believe a word I said, unless you looked at the references at the end of the post. I would not have blamed you a bit. When I first found out about invisible gold in fool's gold, I was all like,

Image shows a kitty head. Cat has its mouth open, one lip crooked in a WTF expression. Caption reads: Whuuuuuuutttt???

But it's completely true and really quite simple! Picture, if you will, a nice, hot hydrothermal fluid circulating through fractures and pores within carbonate rocks. We're in an area like Nevada's Basin and Range, where the crust is stretched and comparatively thin. Magma takes advantage of zones of weakness, and intrudes into the rocks. Water circulating through the crust gets heated by the magma or by metamorphism. Lots of stuff dissolves in it: gold, silver, copper, sulfur, iron, arsenic, and other minerals. But, unlike the hydrothermal solutions that leave behind rich deposits of vein gold, this fluid is unsaturated. The gold isn't going to precipitate out on its own.

Image shows twelve-sided crystals of pyrite embedded in a chalky, off-white rock. One of the triangular faces is gleaming in the light, showing the shapes that create these awesome little pyrite balls.

An absolutely gorgeous pair of pyritohedrons. This is one of the crystal habits of pyrite - it forms these awesome little twelve-sided balls. These are in hydrothermally-altered rock up in Quartzville, Oregon. And yes, there's gold nearby!

This is where arsenic comes in. Pyrite forms from iron and sulfur, but arsenic can take the place of sulfur in the crystal lattice. It's also able to pick up some gold and carry it along for the ride. We're still researching whether an atom of gold can actually replace an atom of iron - we're talking nanoscale particles, here, so it's really hard to determine that for sure. But we know that infinitesimal particles of gold become part of the pyrite. Some of it is incorporated into the crystalline structure of the arsenopyrite. Some particles just get included with the crystal growing around it, sort of like a nonconformist in a crowd of go-with-the-flow folks. Ultimately, we end up with tiny amounts of gold - maybe 100 parts per million, if you've got a rich deposit. It doesn't seem like much. But the gold contained in these deposits actually makes up some of the richest gold mines in the world. The largest invisible-gold-in-pyrite deposit in North America, located in the Carlin Trend in Nevada, also happens to be the most productive gold mine in the country! It's got more than 107 million ounces of gold in it. And we've got the techniques to get that gold out economically enough to make it worth our while.

These Carlin-type deposits occur all over the world. And they're really useful! In Taiwan, the Jinguashi Gold Mine sent its pyrites over to Taiwan Fertilizer, where it underwent calcination. Part of it became sulfate fertilizer; the remains were sent back to Jinguashi and processed to extract their gold. Now that's getting the most of a natural resource.

So there you are. Next time someone hands you some fool's gold hoping to take you for a fool (or just wanting to share their shiny), check it out for invisible gold. You'll know there's a chance you're actually holding some hidden gold in your hand if the sample is silvery-white or steely, and smells like garlic when you smack it with a hammer. If it's a proud, brassy yellow, you may still have a particle or two. How neat is that?

And so sparkly!

Image shows a chunk of pyrite. Most of the crystals aren't well-formed, but there's a good crystal face in the middle that shows pyrite's characteristic striations.

One of my own beloved pyrites, found in a mineral shop in Cottonwood, Arizona. So sparkly! Pyrite has always been one of my favorite things in the world.