30. Colour: Physical Optics and Colour Phenomena

Explore the fascinating world of physical optics in gemstones, focusing on iridescence, thin film interference, and the science behind colour phenomena like labradorescence and opalescence. Perfect for budding gemologists and enthusiasts alike.

key  topics
Physical optics in gemstones
Iridescence and wave interference
Thin film interference in gem materials

Unveiling Gemstone Colours: The Science of Iridescence and More
The Hidden Physics of Gemstone Beauty: A Deep Dive


sound bites
"Iridescence is caused by wave interference."
"Labradorescence is due to the chemistry of feldspar."
"Pearlescence results from stacked layers of aragonite."


Chapters
00:00 Introduction to Physical Optics
02:09 Understanding Iridescence
11:00 Exploring Twin Planes and Their Effects
14:11 The Fascination of Labradorescence and Adularescence
20:03 The Nature of Light and Colour
21:06 Exploring Rainbow Moonstone
22:28 The Allure of Pearls
25:06 Understanding Iridescence in Pearls
28:27 The Mystique of Opals
36:52 Reflections on Quality and Value in Gemstones

 resources
Facet Nation Podcast - https://youtube.com/@facetnationuk


 guest links
Instagram - https://instagram.com/facetnationgemmology
TikTok - https://tiktok.com/@facet.nation.pod
Email - mailto:facetnation@facetnation.co.uk

gemmology, physical optics, iridescence, labradorescence, opalescence, gemstone science, thin film interference, gemstone phenomena

Transcript

SPEAKER_01 0:15

Hello everybody and welcome to another episode of Facet Nation, a gemology podcast. We are your hosts. I'm Lucinda.

SPEAKER_03 0:22

And I'm Simon.

SPEAKER_01 0:24

And tonight we are getting drum roll. Physical. We are going back to the serious business of color, but this time uh we are talking about physical optics.

SPEAKER_03 0:37

And Olivia, Newton, John.

SPEAKER_01 0:39

Yes.

SPEAKER_03 0:40

Okay, good.

SPEAKER_01 0:43

Simon, what is a physical optic? How does it differ from what we've been talking about before?

SPEAKER_03 0:48

Um, physical optics is how light interacts with physical features within a gemstone to alter or enhance or diminish the colour. So this is all related back to colour. This is all part of our colour series. The colour series seems to be going on forever. We're constantly talking about colour. We haven't even started talking about fucking um like valency state of irons and shit yet, which is like the really technical stuff. So we haven't even touched the surface of colour, and we've done about 25 episodes about colour.

SPEAKER_01 1:21

So deeply fascinating, so that's okay. And this one in particular, I think, is a really good one for budding gemologists because there is, you know, there's complex stuff going on, but it does make sense. Like logically, you can connect what you're seeing to what is causing it.

SPEAKER_03 1:37

Yeah, and I suppose some of the things are gonna relate back to previous things that we've said, and then you're gonna sort of start connecting dots back to some of those things, which is which is kind of nice, because you're gonna be like, oh yeah, I understand that. And now I understand how it's being applied here in terms of creating colour effects.

SPEAKER_01 1:56

And the colour effects that we're creating are some of the coolest ones in some of the most iconic gemstones, I would say. There's some heavy hitters in this app.

SPEAKER_03 2:05

Big time, yeah, some light, pretty gnarly looking shit.

SPEAKER_01 2:09

So, with no further ado, ladies and gentlemen, we are going to kick off our physical optics episode with iridescence. Simon, what the hell is iridescence?

SPEAKER_03 2:21

So iridescence is seen on the surface when and it's basically different spectral colours that you see coming back to the eye as the angle of view or the angle of light changes. Yeah. So when you're looking at things that have iridescence, as you move around or as the light moves around, you're gonna see those different colours of the spectrum reflecting off the surface of various things, and we're gonna tell you what those various things are.

SPEAKER_01 2:50

Exactly. It is essentially caused by wave interference, which we've discussed before in our wave theory episode. Uh they are light waves that combine and they either amplify or extinguish. Extinguish? So I'm gonna say extinctify. Extinctify.

SPEAKER_03 3:10

That's that's not a word.

SPEAKER_01 3:11

Annihilate.

SPEAKER_03 3:14

Extinctify, I think, is something that we should um bring into the common parlance and start start saying that. 2026 word of the year. Amplifies or well, it's better than six-seven, isn't it? So, you know.

SPEAKER_01 3:27

Which I've started saying now, because I have a 16-year-old brother.

SPEAKER_03 3:31

Don't don't do that.

unknown 3:32

Six seven.

SPEAKER_03 3:33

No. Um but basically, yeah, it's um either amplify or extinctify, which was our new word, um, particular wavelengths of visible light. Yes.

SPEAKER_01 3:43

Yes, and adults call this destructive interference, by the way. Uh and it basically reduces the intensity. So either they're all working together to make themselves more than they are, or they're working at odds to destroy each other. Much like modern politics, I would say.

SPEAKER_03 3:58

Yeah, and they can like fully destroy each other, or just a little bit extinctify.

SPEAKER_01 4:05

It's a spectrum, guys. Um and of course, speaking of spectrums, as we know from previous episodes, every wavelength will have its own color across the spectrum. So that's just something to keep in mind throughout this episode.

SPEAKER_03 4:17

And to keep just to keep in mind in general, to be honest. Which is why we keep which is why we keep repeating it.

SPEAKER_01 4:24

We're not doing it for fun, guys.

SPEAKER_03 4:26

Yeah, exactly.

SPEAKER_01 4:29

So where might you see iridescence, Simon?

SPEAKER_03 4:32

So iridescence is that multicoloured effect that you see on things like oil slicks, for example. Um, and you can see it in some gem material that exhibit this phenomena. Um, and though that gem material that has iridescence as one of its main features is effectively why it is nice to look at. Otherwise, a lot of these gemstones that if they weren't iridescing would probably look a bit meh.

SPEAKER_01 5:01

Totally. Oftentimes they've got kind of like uh color, like they're not super transparent, but they are iridescent, and that is a ma it's like magical. And what actually is going on goes back to how these materials are constructed, because this iridescence is caused often by what's called thin film interference. And this is all about reflections, but it's reflections from different layers or films. So they can be films of liquid, of solid, they can be cracks and fissures, and they are interacting with the light in a really interesting way.

SPEAKER_03 5:35

Yeah, so when you see oil slicks and they are iridescing, it's because there is a fine layer of there's different layers of the there's like liquid on the on the oil basically, and it's the it's it's it's all about layers. You need a layer for iridescence. Or thin film interference, should I say. As we always imagine light, with all of those waves wriggling around. Um some waves are vibrating in the same direction, as we know, and they will make contact with the surface and reflect away at an equal angle. Yes, the angle of reflectance. Is that a word? Fuck it. It is now. Reflecting um that's better.

SPEAKER_01 6:22

Um This is a gemology podcast, but also we're teaching each other how to speak English.

SPEAKER_03 6:27

How how to yeah, or even just creating a brand new language, which isn't English, it's bollocks. Um anyway, so some of the other waves um that are vibrating in that direction pass through the outermost surface, they refract, and then almost immediately after that surface, bosh, they encounter yet another surface, another layer. Now, those waves might reflect off that one, or they might pass through that one and on to yet another layer. Eventually, though, some of that light will reflect, and when they do, they might re-encounter that first wave, and then if they re-encounter it and everything's sort of nicely in phase, they might recombine. Yeah? So if they're in phase, i.e. the peaks are aligned with the peaks, they amplify, and if they're out of phase, the peaks aligning with the drops, extinction occurs. Very simple. Multiple thin layers of thin films, one after another, as these afflictions occur, uh occur in turn, and these wave amplifications repeat.

SPEAKER_01 7:32

So like a dance floor, like some groups of people are doing great and they understand what's going on and they look awesome and make each other look more awesome. And then as it as people kind of lose the beat and forget where they are, they start to look shit, and then everyone looks more shit. And then it it kind of shifts. I I like to think of it. It's very I like this and I liked learning about it because it feels very human to me. Like it just feels like a really chaotic fun party to be at.

SPEAKER_03 8:00

What are those things called where people just start dancing? A flash mob.

SPEAKER_01 8:03

A flash mob. It's like that, I suppose.

SPEAKER_03 8:10

People joining in the flash mob.

SPEAKER_01 8:13

Exactly. Uh let's say, for instance, that the people joining the flash mob are two waves of the same wavelength. So let's use 460 nanometers in the blue, just as a random one. What what's going on with them?

SPEAKER_03 8:26

So if those two if two of those wavelengths recombine after reflecting off different layers and the peaks align, then that blue wave is reinforced and then appears brighter. This is just constructive interference. So it's brightening the colour that you see. Everything's getting a bit brighter when they when they combine, when they constructively interfere.

SPEAKER_01 8:47

Amazing. And so where we see this in the world is as we've said, on oil slicks and in bubbles. So the oil sits on top of the water to create a separate layer for the reflections. And then in gemstones, of course, so in fractures within gemstones that are filled with gas or liquid, which we know is very common. You can also see it in incipient cleavage, which is an internal cleavage plane where the material might separate and it creates a super thin gap. But then there's a trickier place you might see it too.

SPEAKER_03 9:17

Yeah, so if a gemstone has a surface coating, like in your topazes, like your topaz, I'm not even sure that's a word, man. In your topaz. Some sometimes with the topaz, they put a coating on it to create this effect, and those types of topaz are called mystic topaz. Like they look ridiculous, but it's still quite an interesting effect.

SPEAKER_01 9:43

Definitely. And if you i what I find a lot with mystic topaz is someone will buy it as amethyst or whatever as a teenager and then just never think about it that too hard. And so they just think that that's what like amethyst looks like. But I find them quite fun in and of themselves. They feel very like 70s to me.

SPEAKER_03 9:59

I quite like those um like coatings and the synthetics and things that aren't really trying very hard. I'm I'm sort of alright with that. Unless it's goldstone.

SPEAKER_01 10:12

Yeah, everyone, as we all know. I feel like we should have a bell, and every time you mention goldstone, we should ring the bell. For whom the bell tolls, the bell tolls for goldstone.

SPEAKER_03 10:20

The bell tolls for the blue goldstone. Yeah. Because that's like literally not trying.

SPEAKER_01 10:26

No dwarf. Alright, where else are you gonna see uh thin film interference, Simon?

SPEAKER_03 10:33

So if you've got a composite, so a stone that is made up of two separate gemstones, for example, you might get a small gap where the adhesive that joins the two pieces of material together has degraded and um yep, caused a little gap.

SPEAKER_01 10:49

I love because you're like a it makes me feel like a dog on the hunt for something, you know, those like weapon-sniffing dogs or whatever. You see a little a little of that iridescence, and I'm like, I'm in there.

SPEAKER_03 11:00

Well, if you see if yeah, that's quite an important thing, actually. If you see iridescence in something, then that's there's something to be explored there. So I would be like, oh, there you go, there's a little flash of flash of colour. Let's let's look at that a bit closer, let's get under a microscope, because it might be a fracture, it might be this. And generally, if it's like a cleavage plane or a fracture or something like that, that's got some liquid in it, it's gonna indicate that it's natural. Um, or if you can notice that it's you know two separate layers of two separate things, then that's gonna give you some really important information. So, yeah, that's important.

SPEAKER_01 11:36

Always worth exploring. And also along twin planes, which we're going to dig a little bit deeper into now. Before I send Simon off to talk about twin planes, because they're one of my least favorite things. We're basically just gonna recap and note that anywhere there's a separate layer that light can reflect off one after another, that's where you're gonna be getting this effect.

SPEAKER_03 11:59

Yes, that's exactly it. You need gaps, you need layers. Um, so yeah, let's talk about twin planes. And let's talk about twin planes specifically in reference to feldspar, because feldspar like to grow in layers, which you might remember from when we spoke to Rosie, because Rosie beautiful flaky pastry. So it's basically layers of different types of feldspar. These layers are often very thin, which is good for our thin film interference. So think of it as you've got all these chemicals getting heated in magma, they're reordering themselves, they're exalving into one type of uh feldspar and then crystallizing. And as the crystal cools, these different feldspars separate out into layers. Um, these layers crystallize, integro with each other, so on and so forth. We're building up stacked layers of feldspar. These layers are called lamelee, lamellae. Yeah, however you want to say it, we don't care. We don't we're not gonna judge you. Say it how you want, say extinctify, we don't care. So these layers are called lamellae. Uh the process is called exolution lamellae, and the resultant twinning is called lamella twinning. It's all very straightforward. Except feldspar is actually very complex, mostly for this reason. So imagine these lamellae acting like little stacked mirrors all on top of each other.

SPEAKER_01 13:26

You can also think of it as making croissants. When you make croissants, you do layers of butter and pastry, and they kind of melt into each other and then they become filled with air, and that's why it's all fluffy and flaky. Uh I don't know, you must agree with me on this, Simon, that Feldsbar is the croissant of the gem world.

SPEAKER_02 13:45

Absolutely.

SPEAKER_03 13:46

That's I mean, I think it's in the course notes.

SPEAKER_01 13:50

I'm having deja vu. Do I have deja vu every time we talk about Feldsbar?

SPEAKER_03 13:54

Probably. Okay. I don't know why. We've probably done this episode before. Who knows?

SPEAKER_01 13:58

Exactly. I think we've had this exact convers. I swear to god we've had this conversation.

SPEAKER_03 14:03

I've never heard you say that Feldsboro is the croissant of the gem world, I'm afraid.

SPEAKER_01 14:08

Okay, well, I'm having some wild dreams then. Okay. Back to the topic at hand. Um basically this process, just like making croissant, makes really good stacked layers, and that's exactly what iridescence needs. Uh, we are not going to get into feldspar chemistry today, but do remember that the composition of feldspar um and the balance between different feldspar components is what causes the phenomenon that we know and love. One of my favorite optical effects, I would say. Labradorescence.

SPEAKER_03 14:44

That is a nice one.

SPEAKER_01 14:45

Tell me more.

SPEAKER_03 14:47

So labradorescence in feldspar variety, labraderite. Um, this effect is due to feldspar's fascinating chemistry and how they grow, as we've just been saying. Conveniently, the lamellae in labraderite are typically between 200 and 400 nanometers thick, 200 to 400 nanometers thick, which is basically right in the sweet spot for some visible light interaction.

SPEAKER_01 15:12

Um come on, that's cool. Our luckier way to live in the same world as feldspars.

SPEAKER_03 15:20

So, you know, when you're watching a film and you're like, hmm, yeah, if if this were a documentary, you think to yourself, I don't know if you do this or if anyone else does this, but I often think to myself, isn't it convenient? They're following around the bloke that all of the interesting stuff happens to. I mean, I suppose it's all self-fulfilling, isn't it? Because like we wouldn't be talking about Labradorescence if the millimetres weren't right. And I'm sure there's other things. There's other things that twin and have layers where the where the where it's not right, which is why we don't talk about them. So whilst I agree with you and you're correct, actually it they're kind of like it's chicken and egg type thing.

SPEAKER_01 15:58

That sounds like bitterness to me, Simon. That you do not have lamelle between 200 and 400 nanometers there.

SPEAKER_03 16:05

No, I don't, and I'm actually very jealous. So basically, this range not only explains the phenomenon, but also why certain colours dominate in labradorite. So that that range of nanometer is going to give you your like blue-green shimmer, which is what you see in labradorescence.

SPEAKER_01 16:25

And in the example of Moonstone, which has that moonlit glow known as adularescence. So all of these essences, guys, take notes of them, you're gonna need to know them. In Moonstone, in particular, this effect is due to very fine inclusions, or you could say lamellae that are so thin that they act like very fine inclusions.

SPEAKER_03 16:45

Yeah, so but when I was sort of doing the notes for this, um I I read two completely contrasting things, and it's like inclusions cause it, but it's also the lamelee that cause it, but then it's lam the lamelee acting like inclusions.

SPEAKER_00 17:03

So this is all kind of like I mean, if you if you put some what we're about to say they they seem to they seem to mean the same thing to me.

SPEAKER_03 17:13

They're all kind of the same thing, and it's different different reasons. But basically, what we're going to now describe to you is kind of a a bit of this and a bit of that, and it's it's kind of right, and it's kind of like could be could be argued that it's not right, but and that's the same with all sorts of things, isn't it?

SPEAKER_01 17:30

So welcome to the world, guys. Strap in. So I'm going to talk to you about exolution again.

SPEAKER_03 17:35

So we've got extremely fine exolution lamellae of albite within or with a clays, and that's creating scattering. And that is acting as microscopic particles or inclusions where the ultra-thin lamellae is causing multiple micro reflections all at once, which then combine to create that mysterious-looking haze. So if you think of clouds in the sky, um they are effectively water particles which are causing light to scatter and creating a literal cloud. Um, it's kind of a similar thing. So if light's having lots of micro reflections off lots of different things, then that's going to cause scattering and then haziness and then clouds.

SPEAKER_01 18:18

For this next part, guys, go out into the sunshine, look up at the sky, because the size of the particles or inclusions matter, and we're going to use the sky as an example to show you how and why. So, due to the size of small particles in the atmosphere in a process called Rayleigh scattering. Okay, so in this process, the particles are slightly larger and similar in size to the wavelength of light. Am I to do am I making sense? I just changed the cadence on that.

SPEAKER_03 18:49

I think I think I think you're sort of making sense.

SPEAKER_01 18:51

I'm making some some sense. We're talking about why the sky is blue.

SPEAKER_03 18:55

The sky is the sky's blue because of small particles in the atmosphere. Um yeah. And that that that is down to Rayleigh scattering. Um, but when the particles are slightly larger and similar in size to wavelength of light, the result is tyndle scattering, which means shorter wavelengths are scattered. The more abundant the particles means the Tyndall blue colour appears stronger. So there's Rayleigh scattering, Tyndall scattering, like to be to be perfectly honest. I don't know loads about Rayleigh and Tyndall scattering, but they're things. That's why the sky's blue. Let's move on.

SPEAKER_01 19:31

So the lamella particles in Moonstone are such that they also produce a blue colour. You guys will know it if you've ever spoken to the colour.

SPEAKER_03 19:38

That's why we're talking about Tyndall scattering, because it's more or less the same sort of thing. Yeah. Thank you. It's an example.

SPEAKER_01 19:46

So all of the lamellae particles in the moonstone are scattering shorter wavelengths, and so you get this characteristic blue, almost glow, that is um being transmitted forward to the viewer. And this is how and why adularescence occurs with both reflected and transmitted light. So the size of the gaps, the layers, yeah. Um it all affects the color and the intensity of these color phenomena. So when you see something like this, it's happening because of the physical structure of what you're looking at. Even if you can't see anything, like when you look at the sky, there's loads of particles up there just through my pen.

SPEAKER_03 20:22

Loads of particles up there, and that's why the sky's blue, because if there was nothing there, it wouldn't be blue, would it? I don't know what it would be. It would just be the night sky.

SPEAKER_01 20:31

Yeah, exactly.

SPEAKER_03 20:32

So it'd be like black, like space.

SPEAKER_01 20:35

That just present a real existential chill down my spine.

SPEAKER_03 20:38

Yeah, but the sky isn't but the sky isn't black, is it? It's blue. And that's could due to dindle and railly scattering.

SPEAKER_01 20:44

Shout out to the lads. Um we watched Project Hail Mary the weekend. I loved it. Highly recommend to all the dorks in the audience.

SPEAKER_02 20:53

I don't know what that is.

SPEAKER_01 20:54

It's got Ryan Gosling in it, it's about the end of the world. But it's in space, which is why I think of it.

SPEAKER_03 20:59

Got it. Okay. Yeah, no, I do know what that is. That does look good.

SPEAKER_01 21:02

Alright, well tell enough about that. Tell us about Rainbow Moonstone. Quote Yes.

SPEAKER_03 21:07

A particular gem variety that is on vogue at the moment, you might say, is the Rainbow Moonstone. Now, Rainbow Moonstone is actually a variety of colourless labraderite feldspar rather than true orthoclase moonstone.

SPEAKER_01 21:21

They just wanted to make money off of it. I was a hater, and then I saw it faceted in.

SPEAKER_03 21:26

Moonstone is orthoclase. Rainbow moonstone is labraderite. Let's get that clear. Um the layers can sometimes be a bit thicker and therefore expand up to the higher wavelengths of colour beyond the blue and into the green and the orange. So the colourless body colour makes for a mistier overall effect and produces these beautiful results of the rainbow colours and the haziness and the mistiness and the scattering and all of that stuff. Um Yeah. That's rainbow. Moonstone.

SPEAKER_01 22:01

And they're very interesting. They're starting to facet them in different ways now. Um, they're not a they're not a reliable friend, I would say, but they are incredible when they get it right.

SPEAKER_03 22:10

Yeah, they do look lovely.

SPEAKER_01 22:12

Yes. What else looks lovely? Pearls. And everything that we're talking about actually relates to them directly because this idea of the size of layers in correlation with wavelength is why we get the effect that is called pearlescence. Now, this episode is about colour and it is not about pearls. Trust me, it is on my list. But pearls grow in a way that creates this interference effect as well. So the iridescence in pearls is very subtle. Uh, but as a teaser for a future pearl episode, Simon, you love pearls, it says here.

SPEAKER_03 22:46

I actually do. I've got I've had like a big appreciation of pearls in recent in recent times. And actually, when you see a really nice one, you're like, hmm, yeah, that's actually worth looking at.

SPEAKER_01 22:58

No, I totally agree. Um, and so basically, pearls are made of platelets of aragonite, which are stacked and layered in a matrix of conchulin. Now, conchulin is an organic protein secreted by a mollusk, and it's basically what allows a pearl to form. It is the like sticky spit that keeps all of these platelets together. And if you've been listening, you'll notice that the key thing here in terms of interference is due to the fact that the aragonite plates are stacked. Iridescence means stacked layers, so we are in a really good place to see this effect.

SPEAKER_03 23:34

Stacked layers. As long as we've got stacked layers, we can get some iridescence. Yes. So this aragonite builds up like brickwork, brickwork around the pearl's nucleus. Light reflects off these various layers at different depths and various thicknesses. The thickness of this coating, known as Naker, is dependent on the sea temperature during growth. So, depending on when the pearl is harvested, the outermost layer layer can be either thick or thin. The effect we're looking for prefers the outer layer to be thin for fairly obvious reasons that light can get through to the inner layers a bit easier because if it's thinner, the light's going to get through. And that's going to produce more reflections and more subsequent interference. Too thick or too thin, and it has a knock-on effect to the interference quality. So the process of thin film interference is pretty much exactly the same here, but with pearls though, we have yet another factor in play.

SPEAKER_01 24:30

That is true. These platelets are not perfectly wedged up against each other. So think of a drunk bricklayer. Or Simon's favorite thing in the world, a dry stone wall.

SPEAKER_03 24:43

I do really love a dry stone wall. I have a mug with a dry stone wall on it because a mate of mine bought it for me, knowing full well that I'm a big fan of a dry stone wall.

SPEAKER_01 24:52

Interesting. We'll dig into that in therapy later. But the what we really need to understand about dry stone walls is that there are gaps. So gaps and layers. We know what the layers are doing. What about the gaps, Simon?

SPEAKER_03 25:06

Well, what happens to light when we try and shove it through a tiny crack, tiny gap? I'm waiting for the answer. Yeah, well, you should all know it's diffraction. Light splits into spectral colours as the wavelengths travel through the gaps at different times, causing them to spread.

SPEAKER_00 25:24

Exactly. So let's go now go back. Oh, go on, go on. Tell us.

SPEAKER_03 25:28

So basically, when you look at a pearl, and we're talking fine quality pearls here, we're not talking like the shit ones. Um, what you want to do is you move it around, you look at it from different angles, let the high light hit it from different angles, and observe that multicoloured sheen and bright mirror-like reflection. Now, this is a combination effect, all because the host mollusk is crap at building its aragonite defence wall. But also at the same time, it's amazing at building its aragonite defence wall because it's creating this effect. So by being crap, it's actually doing something pretty special. So, like, is it crap or is it doing it on purpose? Who knows? It's probably not doing it on purpose. No, it's probably not doing it on purpose. It's it's it's simply trying to protect itself, so it's not doing it on purpose, but it would be nice if it was like, I'm just gonna do this a bit shit so that so that we create this nice looking pearl. Um, so basically, this combination effect is known as the Orient. And when you see it in action on a good quality pearl and understand what's actually causing it, you have this brand new appreciation for the majesty of the pearl, which is effectively why I have a new appreciation for the majesty of the pearl. Um, white okoya or south sea pearls have a nice pinkish shimmer in the best examples, and Tahitian or black pearls, they're actually silvery grey rather than black, but whatever, give a greenish peacock feather-like sheen in the best examples.

SPEAKER_01 27:04

They're beautiful. And when you start really being able to appreciate them, they're wonderful. You're about to you you say that you want to repeat something, Simon.

SPEAKER_03 27:15

I'm gonna I'm gonna repeat something here. These effects are only seen in very high quality pearls. The layers need to be the ideal thickness to produce the right colours, and the platelets need to be ordered in a certain way. If this criteria is not met, or even if it's just slightly off, everything becomes a bit milky and dull. So judge a pearl on those colours you see, and literally by how clearly you see the reflection of your face in the surface. That's when you know that pearl is working hard and can confidently say that this has fine luster and orient. And then you'll sound like a pearl expert.

SPEAKER_01 27:51

Indeed. Such a unique gemstone, of course, as it combines luster, so those surface reflections, thin film interference, diffraction, and scattering, all in varying amounts, but playing their own part to achieve this beautiful pearlescent effect. So, I mean, that was very poetic.

SPEAKER_03 28:07

You can't say that about many other things, though, can you? That does all of that shit.

SPEAKER_01 28:12

Let's talk about another fucking weird one. So we've spoken about gaps and layers, so let's talk about all of those things combined, how they all work together in a totally different material, which is opal.

SPEAKER_03 28:35

Um, they're a delicate soul and quite mysterious really in how they're formed. Um, but what it's composed of and yeah, but basically what we do know is like you know that dorky kid at school that has gaps, not between its teeth, but between spheres of silica. So yeah, an opal is the dorky kid at school, the gappy teeth. Um, and in those gaps, um so these so these silica spheres creating gaps by grouping together silica spheres in their own little sections or cliques, if you like. So let's go back.

SPEAKER_01 29:17

The opal is the high school, and the silica spheres are the pupils.

SPEAKER_03 29:22

Yes, well done. You put that into something a bit more sensible than what I was saying. The opal is the high school. We've got your dorky kid with the gappy teeth, yeah, as it's sort of like the the overbearing master here, because we are talking about gaps in general, but there's only gaps, and there's different size gaps because of the cliques in the high school. Um, so you've got your big guys, you've got the jocks, I suppose. They're gonna huddle together in one area over here, and then you've got the smaller, slightly intellectual kids in another area, they're going to have their little section over here. Um, the goths and the alternative kids might be hiding away, and then the popular kids. Well, the popular kids sort of show up whenever they want. It's on their terms. They do, you know, and when they do show up, you're very grateful that they're and you're like, Oh, what's the popular kids? So, yeah, try and think of these little groups huddling together. And I'm gonna let Lucinda take over because I'm not making a lot of sense.

SPEAKER_01 30:22

So, all of that self-segregation between social classes and can and groups uh is in service of what we call play of color. This is an amazing phenomenon, and it's a simple name that really explains exactly what's happening. So the colors of the rainbow playing around and across the surface of the gemstone when a light source andor the angle of view moves. So this is when you think of opal and you're turning it, and sometimes there's green flashes of color, and sometimes there's red, and sometimes there's blue. That is play of color. And what we're really talking about is brag diffraction. So parallel planes in the 3D periodic lattice causing diffraction and constructive interference of reflected waves. So in Opal, this is due to the spacing between the spheres, all of these different groups of kids, leaving little gaps between each other. That's what is causing this, and it creates these planes and causes these different colors. So it's basically a 3D diffraction grading.

SPEAKER_03 31:24

Exactly it is. Yeah. So Opal is structured in these pac patches containing these silica spheres, and they group together in clusters of uniformly sized spheres. Now the size of the spheres in each grouping varies throughout the material, and it's the size which determines the particular wavelengths that can be diffracted through the gaps between them. Now the small spheres, the small intellectual kids, let's say have small gaps and diffract the shorter wavelengths of blue and violet. This is why most oil pools have blue play of colour, because you have more of the the small spheres, um, because it obviously takes longer for the larger spheres to form. Um the larger spheres, which are the jocks like the you know, the the big football playing kids, um they're much rarer because you know it's difficult to be an athlete and not difficult to be intelligent. That doesn't make any sense.

SPEAKER_01 32:20

I'm enjoying this metaphor. Um it's true to my experience of American high school.

SPEAKER_03 32:27

Yeah, well, it's like you know, all the TV shows and movies that we used to watch as a kid. But yeah, it's much more it's much more difficult to be an athlete than it is to be intelligent. Not true. But anyway. Those larger groups with the larger gaps basically allow the longer wavelengths to diffract right up to the biggest ones, which allow the yellows, the oranges, and the reds. So when we say bigger, we're generally talking bigger than 250 nanometer spheres in diameter.

SPEAKER_01 32:56

Yes, and we've talked about this in reference to waves before, trying to give all these different wavelengths and the colors they produce some personality. Smaller spheres, 200 nanometer spheres, that is blue light diffraction. The blue is very uptight, and everything is gonna send it into a freakout. 350 nanometer spheres, this is where you're getting into green light. These are all your like well-adjusted people, they're not too you know stressed about anything, but they still will react. And then 320 nanometer spheres, this is red light. These uh, these guys are chill, like they'll refract a little bit, you know, it's all good. Um and it's warm.

SPEAKER_03 33:35

And it's that's quite that's quite rare. Um, obviously, all of these um things depend on spacing and the viewing angle, but as a general rule, this is pretty much what's happening here. Like that those size spheres are going to create those size gaps, which is gonna cause that type of diffraction.

SPEAKER_01 33:52

Yes, but don't get it twisted, guys. You are not going to be able to see all of our lovely high school students, uh, at least without a scanning electron microscope. But the colours will tell you who's there, how big they are, whether they're getting along.

SPEAKER_03 34:09

Yeah, so an opal with play of colour containing the whole colour spectrum from blue to orange and red is considered much higher value because you know getting the jocks to come to the party is tricky. Um, due to the beauty of this going. No, probably not. Like no, it's not, but whatever. Um so a black opal that displays red flashes is literally the pinnacle of this.

SPEAKER_01 34:36

So we've gone to the pinnacle. Now let's go to the pit. Uh, lesser quality opals, the kind of sad guys, they still have something, they have an essence, they have opalescence, which is a scattering effect like you see in Moonstone. And this is due to the spheres not huddling together in their peer group. So, actually, the more progressive uh and happy a social system, the less interesting the optical effect that you're gonna see. And that's just how life works, you know? So the chaos is basically like the jocks turning up to the nerd's chess club. The unevenness of the size of the sphere is usually above 700 nanometers, causes this kind of cloud-like appearance, and the opal will appear milky and kind of generally white. You've probably seen these opals. These are known as common opal, whereas an opal with play of color is called a precious opal. And this opalescence can be seen also in milky quartz, some chalcedonies, and in corundum. It's basically the same effect, which is light scattering without direction and no differential in the wavelengths being scattered. Opolescent, often pastel like sapphires are becoming increasingly popular. I actually love these guys, they've got like a really fine glow to them, and the scattering in this instance is usually caused by poorly oriented rootile needle inclusions. Long live the rootile needle.

SPEAKER_03 35:59

Yeah, I mean like they're really nice, and people are buying this. This stuff used to be um heat treated out, we dissolve all this rootile, but actually people are enjoying the effect that the rootile is causing, and people like sort of muted pastel colours, don't they? And this is gonna achieve it.

SPEAKER_01 36:21

And also, so when you hear somebody saying like their sapphire is really milky, that's what this is talking about. It's like this beautiful soft effect, it's very individual, and also which I think is only gonna get more important, it's a sign that your so your stone is natural, right? Your stone's been through some shit.

SPEAKER_03 36:40

Exactly. Yeah, we're not we're not sort of like we're not making it look like something it doesn't want to be. It's being who it is, and we're respecting it for that, which is lovely.

SPEAKER_01 36:52

We've really we've learned a lot this episode. I feel like we've got made some really salient points about the world.

SPEAKER_03 36:59

Yeah, absolutely. And uh I hope I hope you all agree. Let us know what friendship group you exist in, or if you're just one of those people that held to that. I'm getting involved. I want to be opalescent, I want to be milky. I like being hazy. Huh?

SPEAKER_02 37:15

What group were you in?

SPEAKER_03 37:20

I don't I don't know really. I was probably one of those sort of like I used to sort of skateboard and wear hoodies and not talk to anyone.

SPEAKER_02 37:29

Listen to the city.

SPEAKER_03 37:30

So probably the probably the weird group that was hiding. I was also sort of friends with like the clever kids as well. Yeah, but not not the athletes, because that's I've never played basketball, but I'm English, so there was only about three people that knew what basketball was.

SPEAKER_01 37:47

You're tall, so like showing.

SPEAKER_03 37:50

Yeah, exactly. Yeah, everyone else was small. So it was easy.

SPEAKER_01 37:55

Yes, guys. Well let us know which nanometer light you would be refracting.

unknown 38:01

Yes.

SPEAKER_01 38:01

I'm green for sure. For sure. Uh yeah, and that covers the key elements of these concepts. Hope that was helpful.

SPEAKER_03 38:10

Yeah, I kind of hope I kind of hope it was helpful. It might be. I mean, that like sometimes it's much easier to remember things when someone's brought it to you in a nonsensical manner because you're like, that was madness. But it sticks in your mind, doesn't it? If someone's just going to tell you all of this stuff in a scientific way, it's boring. And we're trying to help you remember things by making it a bit weird.

SPEAKER_01 38:33

Exactly. What croissant are you?

SPEAKER_03 38:35

What croissant are you? What friendship group are you? How big are your gaps? Let us know.

SPEAKER_01 38:41

You want to know them all. Yes, we do. You can let us know primarily on Instagram or at facetnation gemology. Uh, you can also let us know on TikTok. Oh shit. Sorry, my cat just like did a crazy flip.

SPEAKER_03 38:54

What's our TikTok name? I think it's at facet.nation.pod.

SPEAKER_01 39:01

Nice. Find us there. You can also email us. It's probably gonna be Simon that will see it because I don't know how to use Microsoft Office or whatever. Uh, and that is facetnation at facetnation.co.uk. Yes, it is. Simon has some news for you.

SPEAKER_03 39:18

I have some news for you. Now um it's Easter weekend next week, and um seeing as we like chocolate. This week is a cigarette. This week, sorry. Yeah, well I mean that this weekend coming is Easter weekend, and um we are big fans of chocolate here at Fastet Nation and big fans of having four days of not going to work. Um even though um, well, my uh I should be going to work on Saturday, but I'm actually not, so I do have four days of not going to work.

SPEAKER_01 39:47

I'm gonna be on holiday with my mom.

SPEAKER_03 39:49

And exactly these sorts of things. People do things over this bank holiday weekend. So what you're not gonna want to do on Monday evening is listen to us because well, you won't be able to, because we're not doing an episode next weekend, because it's Easter, and we're gonna we're gonna have a little Easter, we're gonna have a little Easter break. So if you miss us, then let us know because it's nice to be missed and loved. We'll still be checking the socials because we're still here if you need us, but we're gonna have a little break, eat some chocolate, and then we'll be back with you with a banging interview the week after.

SPEAKER_01 40:23

And we wish you guys all a very happy Easter if you celebrate, or a very happy long weekend, if that is your chosen uh celebration of the year. It's probably mine. I'm gonna sleep so hard.

SPEAKER_03 40:35

Uh I hope you all have a wonderful time.

SPEAKER_01 40:38

And we will see you soon.

SPEAKER_03 40:40

We will indeed. Thanks very much. Bye.

SPEAKER_01 40:42

Bye. Can't Stephen say bye. He doesn't speak again.