This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Gemstones have value not only because they're beautiful but also because they're rare. What exactly is a gemstone, and what makes them so rare?

Lee Groat is a professor in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia. He studies the mineralogy of gemstones and also works in the field, having conducted surveys of parts of northern Canada looking for emeralds. Lee Groat, welcome to Geology Bites.

Lee Groat: Thank you.

Oliver Strimpel: Is there a geological definition of a gemstone?

Lee Groat: Not really. The definition is more set by the market. It's what people will pay for, for its beauty and adornment value. Gems are crystalline materials that contain several different elements. It's not really a gem, but a simple example is quartz, which has two elements, silicon and oxygen.

Oliver Strimpel: Are there some gems that are just a single pure element?

Lee Groat: Well, the best example is diamond, which is just carbon. But almost all gemstones are combinations of elements. For example, gem corundum, sapphire and ruby, is aluminum and oxygen, and a small amount of a chromophore, or a transition element, that gives it color.

Oliver Strimpel: Just to explain, corundum is the crystalline form of aluminium oxide, and it occurs in two gem varieties, ruby and sapphire. So in crystalline rocks, these element combinations occur as minerals. Are gemstone crystals made of a single mineral only?

Lee Groat: Most are single minerals. You can have an ornamental stone, of course, that is a combination of minerals, but most of what we think of as gemstones are single species.

Oliver Strimpel: So you mentioned a chromophore just a moment ago. That, I presume, is a trace amount of some material, some element, that makes a profound effect on the color of the mineral. How does that work?

Lee Groat: They work by changing the absorption spectrum of the mineral. And this is because chromophores can have an effect on what we call the crystal field of the mineral. And for example, a small amount of chromium in corundum can change the color to red, but that small amount of chromium in a beryl will give you a green color, give you an emerald. So in both cases, the chromophore is chromium, but the crystal structures are very different, and so the type and distances of atoms around that chromophore lead to different absorption spectra, and that's why we get the different colors.

Oliver Strimpel: Do these chromophores actually replace some of the major elements in the crystal lattice?

Lee Groat: In most cases. For example, in corundum, chromium replaces aluminum. But there are cases where you actually have a chromophore in a site in the crystal structure that's usually empty. For example, we fairly recently determined that the color of aquamarine, which is blue gem beryl, is due to interaction between iron replacing aluminum in the crystal structure and another iron in a site that's usually empty. And those two iron atoms trade an electron back and forth, and that leads to absorption, an absorption curve that lets blue through and absorbs other colors, and that's how we get aquamarine. But that took a long time to sort out, because the amounts of iron are vanishingly small and very difficult to measure.

Oliver Strimpel: So are we talking parts per million? Just how trace are these chromophores?

Lee Groat: Well, they're very trace, usually more than one part per million. In the case of, say, emerald, you might be looking at about 0.1 weight percent chromium oxide. Yeah, it can be very, very small amounts.

Oliver Strimpel: As I mentioned in the introduction, gems are rare. What makes them so rare?

Lee Groat: They're rare because the conditions to form them are unusual. And the example I usually use is emerald. Now, emerald is the mineral beryl, so you need beryllium, and the chromophore that gives the green color is usually chromium. And beryllium and chromium travel in very different geochemical circles. Beryllium is usually associated with granitic intrusions, and chromium is an oceanic element, associated with basalts and rocks like that. And so usually they don't come into contact. So we need unusual circumstances. Like in Yukon Territory in Canada, we have chromium-rich rocks which are part of terranes that were ancient island arcs that were scraped onto the North American continent, and then these were later intruded by granites. And there are a couple of places where we have emerald occurrences, because the beryllium and chromium could come into contact.

Oliver Strimpel: When we were talking earlier, you also mentioned an example where we had these emeralds, but we couldn't really find any granite nearby.

Lee Groat: Yeah, that's the case in Colombia. And the world's best emeralds come from Colombia, and there's over 200 occurrences and about a dozen producing mines, but there are no intrusions. And studies have suggested that all the components needed to make the emeralds come from the host black shale. So the chromium, the beryllium, the oxygen, the silicon, the aluminum, these all came from the black shale, and they were scavenged by fluids, by brines.

Oliver Strimpel: Just to explain, shales are fine-grained sedimentary rocks, and black shales are shales that are rich in carbon.

Lee Groat: Yeah. So it's a very unique and different type of genesis. And for a long time, it was thought that Colombian-type emeralds were only found in Colombia, that these conditions didn't happen anywhere else. But in 2010, there was an occurrence in the Northwest Territories in Canada which appears to be similar. So that's one thing we're doing: we're keeping our eyes open and developing ways to explore for Colombian-type emeralds in places where we expect they would be, like northern Canada.

Oliver Strimpel: Apart from bringing together elements that don't normally cross paths, are there other kinds of rare geological conditions that give rise to gems?

Lee Groat: Yeah, definitely. We've been working on red beryl occurrences in Utah, and this is where the manganese, which is the chromophore in this case, is in lava flows, and these beryls are forming in basically gas bubbles in a relatively recent lava flow. So that's one example. Others, for example opal: they form where there's lots of silicon, and in the best precious-type opal, you have spheres of silicon and oxygen, and the interaction with light is where you get the opalescence that we see. There's other examples. I mean, probably the best one is diamond. Diamonds form usually from kimberlites, and kimberlites are very rare occurrences.

Oliver Strimpel: To explain, kimberlites, which are named after the town of Kimberley in South Africa, where they were first discovered, are igneous rocks containing diamonds. These were formed at great depths, where high temperatures and pressures prevail, and were brought to the surface via vertical structures called kimberlite pipes.

Lee Groat: That's correct.

Oliver Strimpel: Often in the final phases of intrusion of igneous rocks, such as a granite body, you get pegmatite veins, which can contain very large crystals. Do they sometimes carry gems?

Lee Groat: Definitely. Pegmatites are really interesting. As you said, they form at the final stages of crystallization of a granitic melt, and so they are water-rich, and they also tend to concentrate incompatible elements that wait until the last minute to go into crystals. When pegmatites escape from a parental granite and we get pegmatite veins, they cool very quickly, and it turns out this is why they have large crystals. It was always thought, and this is what I was taught as an undergraduate, that the crystals are large because they were slow-growing. Well, recent research has shown that this is not the case: that they're large because the pegmatite cooled very quickly, and so it was undercooled, with very few nucleation sites, and so the crystals were able to grow to quite large sizes. And often in pegmatites, we also have vugs, or gas bubbles, and those bubbles can contain very gemmy crystals, and they're always good places to look. For example, at the Himalaya Mine in Southern California, we get beautiful tourmaline crystals growing into vugs in the pegmatite. So they're definitely a great target.

Oliver Strimpel: All these special geological circumstances that gave rise to gems seem very local, on the scale of perhaps a few meters or kilometers. What about large-scale metamorphic belts? For example, the rubies that are found in Myanmar, which is in the Mogok metamorphic belt on the eastern edge of the Indian plate. Are there gems that result from larger-scale tectonic interactions?

Lee Groat: Definitely. The one you just mentioned is an indication, or a hint, that at one point we had continent-to-continent collision. That's what we need to get the high pressures and temperatures to form the corundum gemstones, ruby and sapphire. Another example is jadeite. And we now know that jadeite forms in response to subduction of oceanic plates, and that the elements needed to make the jadeite are subducted to depth, and then we have a melt created with the right elements to make the gem material jadeite. So when we find jadeite, or we find sapphire and ruby, those are strong indications that at one point we had tectonic activity, either subduction or continent-to-continent collision, even if many of the other signs of those things are lost. And this has led to them being dubbed plate tectonic gemstones. And there's work ongoing right now on other gemstones to see what they can tell us about prior large-scale geological events.

Oliver Strimpel: Wow, that's fascinating. So these are tectonic traces when all the other continental or subduction evidence is gone away. The gems are the survivors.

Lee Groat: That's correct, yeah.

Oliver Strimpel: OK. So let's say if one of the unusual circumstances required to form gemstones arises, are we pretty much guaranteed to find gemstones? To use your first example, if we bring beryllium and chromium together, are nice emerald crystals always going to form?

Lee Groat: No. To get a gem, you also need conditions that allow a crystal to grow in a perfect way. And often you'll grow a crystal and then there'll be post-crystallization deformation. And this actually happens with a lot of emerald occurrences, where the crystal is broken up or shattered after it's crystallized, by tectonic forces. And so to get a gem, your crystal needs to have been protected somehow. And this can be through being at fold hinges, or growing in pockets with micas, or in a gas-filled vug in a pegmatite. That crystal has to be protected somehow from deformation. And so a number of things have to come together to give you a gem.

Oliver Strimpel: Just to explain, a vug is a smallish cavity inside a rock, most commonly formed within cracks and fissures opened up by folding and faulting. So presumably gems have to be protected from deformation not only while they're growing, but also throughout subsequent geological history, till we find them.

Lee Groat: That's correct. And there was an important emerald discovery in the Yukon Territory in the late '90s, and everyone hoped it would become a mine: beautiful-looking crystals, up to several inches in length. But when you start cutting them, you realize that they've all been shattered. They've all been broken up by post-crystallization deformation. So what initially looked like a very promising deposit didn't turn out at all. Lots of material, beautiful color, but the largest stones ever cut from that deposit are about two and a half carats, and the market looks for a consistent supply of larger stones.

Oliver Strimpel: So was it near some large faults, or what made it so deformed?

Lee Groat: The emeralds crystallized about 100 million years ago, but they're only 15 kilometers from a huge fault, the Tintina Fault, that cuts right across the Yukon Territory, and that was active up until about 75 million years ago. Yeah, I mean, there was obviously local deformation associated with that fault, and that's why the emeralds are shattered, broken up.

Oliver Strimpel: Do you think that, given how rare each of these gem-forming geological settings appears to be, that we've already found most of the sites in the world that are conducive to growing emeralds?

Lee Groat: Oh, good heavens, no. Much of the world, I think, is underexplored with respect to, especially, the colored gemstones. And if we look at countries like Canada and Russia, there are huge areas with a lot of forest cover, glacial till, et cetera. Yeah, they're very underexplored. Also, people doing exploration are usually looking for metals, not looking for gemstones, and they probably aren't looking, and really wouldn't know where to look. And so one thing we're doing is creating exploration guidelines for the colored gemstones. And an area I've mentioned before that I think has great promise is northwestern Canada, where we have a handful of emerald occurrences, and where I think that there is great potential for more.

Oliver Strimpel: When we were talking earlier, you said that it's no accident that most of the places where we've already found gemstones happen to be in very populated areas, because that's where people are, and they're looking for all kinds of stuff. So if you go out, as you obviously have, into the middle of nowhere, you're saying we could be finding just tons more.

Lee Groat: We could be. The question is how to explore. And yes, we have very low population density, so people are not finding these things by accident. And we often use stream sediment surveys to explore for metals, and we're looking at how we could identify the minerals in a stream sediment sample to help us explore for gems. And another thing is using drones. My students have been using drones to successfully explore for peridot, which is gem olivine, in British Columbia, and they say that what used to take three weeks now takes two days with a drone.

Oliver Strimpel: So things like this might help us evaluate large areas relatively rapidly. But you still apply some kind of a geological filter up front, where you're looking for these circumstances that you just described to me.

Lee Groat: Definitely. In northwestern Canada, we have large expanses of black shales that look very similar to what we see in Colombia, and the discovery in 2010 of the Colombian-type emerald in the Northwest Territories is a real hint that the conditions are right, that we should go looking for more there.

Oliver Strimpel: Any hidden subduction zones or continental collisions lurking up there in the Northwest Territories or the Yukon?

Lee Groat: Yes. Well, I don't know. Geologically, it's a real mess. There's lots of terranes, and this is an active subduction zone today. So who knows what's possible? There's a lot of fragments of ruby and sapphire that have been found in western Canada, and so one of my former students is following up on that.

Oliver Strimpel: For my last question, I want to ask you about the most impressive gemstone you've ever seen in the field, and what you know about how it formed.

Lee Groat: I really had to think about this one, because I've seen some very impressive things in the field. But I think the most impressive was cobalt-blue spinel on Baffin Island. It's a bright, royal blue color, and it's in the middle of nowhere, and you can see it meters away, these blue crystals in a white marble outcrop. And as far as I know, they're still up there. But that was really, really impressive. We did a lot of research to find out how they formed, and our conclusions were that the cobalt, the chromophore, must have been in the original sediments, which have gone through a very tortuous metamorphic history to give us these spinels.

We were up there primarily to study a sapphire deposit, which is very close to the cobalt-blue spinel; in fact, it's about a half-hour walk. And the sapphires were discovered in 2002 by some local brothers who were out hunting and stopped for a break and saw these blue stones in the rock, and they're beautiful blue sapphires. And I went up there soon after that, and it took about 15 years to sort out how they formed. And again, it's a very tortuous metamorphic history. But we did do that, and we finally got together our paper. We were going to publish it in the summer of 2017. And I was home that summer for a week or two, and I turned on the national news, and there was Canada's governor general presenting a brooch to the Queen, Elizabeth, on the occasion of her Sapphire Jubilee. And the brooch was made of Canadian diamonds and these sapphires. And the governor general actually even called them Beluga sapphires, which is what we had been calling them. It was just total coincidence that our paper was going to be published later in the same week. There was quite a bit of media attention to the fact that the same week this brooch was being presented to the Queen, the scientific study showing how the sapphires had formed was being published.

Oliver Strimpel: Lee Groat, thank you very much.

Lee Groat: Thank you.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.