- Hey, smart people, Joe here.

(soft music) I need to talk to you about a bird.

- Our biggest wow factor is probably this crimson topaz here.

- [Joe] That's not real.

(crosstalk) Hold on a second.

Did you see that?

- The camera.

- That looks fake.

I also need to talk to you about a bone.

- [Jeffrey] So this is an opal.

This is actually a whale bone from Australia that in fact has been opalized.

- Every part of what you just said is ridiculous.

That's probably the coolest fossil I've ever seen.

And we need to talk about all these beetles.

(Joe chuckling) It turns out you can order dead bugs off the internet.

I've been ordering a lot.

All of these have one incredible thing in common and it is nature's greatest color trick.

In past videos, we've taken a deep dive into some really mind-blowing ways that nature makes color.

We've looked for the blackest black.

We've asked why blue is the rarest of all colors, but I've been saving this color trick until now because, well, I think it might be the best one.

(soft intriguing music) This is a phenomenon called iridescence.

It means rainbow colored.

We find iridescence in loads of places, here, here, here too, and there, also there, even in that dirty puddle out in the parking lot.

But why?

Now, color has many functions in nature, sometimes it's for getting attention, sometimes for staying concealed and sometimes for reasons we don't understand.

But iridescence is uniquely mind blowing because the colors that we see aren't really there.

They come from a weird trick of physics.

To figure out how it works, I asked a beetle expert, a bird expert and a rock expert to show us some of nature's most incredible examples of iridescence.

It turns out, if you go to a huge awesome museum, like the National Museum of Natural History in Washington DC, they have entire rooms full of awesome colorful stuff to show you.

We started in the bird wing.

(crickets chirping) Get it?

Bird wing.

Okay, sure, so peacocks get all the attention when it comes to iridescent plumage, but I think that the fanciest rainbow feathers belong to the smallest members of the living dinosaur family, hummingbirds.

- They're gorgeous, unbelievable.

Our biggest wow factor is probably this crimson topaz here.

- [Joe] That's not real.

(crosstalk) Hold on a second.

- [Sarah] Gives you a flash at the camera.

- Did you see that?

That looks fake.

It looks like somebody made a hummingbird in a lab and said, "This would be cool."

Why do hummingbirds have these amazing colors?

- So hummingbirds, we think, it's always a we think in science, right?

We think that hummingbirds have these amazing colors because largely they're using them to attract mates, picking who they might wanna have offspring with based on who is the prettiest because pretty might indicate best genes, best ability to find food, best ability to care for offspring.

- You don't have a lot of parasites crawling all over you, which I know is a big problem in dating.

These look really cool.

There's like purple and green and everything.

- These are beautiful.

The different body parts of these birds have different iridescent colors on them.

You get these brilliant greens on the body and these really beautiful roses and violets on the tails.

- I mean, I can see how this would get attention in the hummingbird dating community.

What is happening inside of these feathers that helps create these colors?

- It's just a little bit of a trick of physics.

There are three things that make up the basics of this iridescent color in these hummingbirds, melanin, which is the same pigment that colors your hair, keratin, which is what makes up the feather and also similar to your fingernails, and air.

- The way that light dances off of hummingbirds doesn't come from the color of the pigment in those feathers.

It comes from how the feathers are built.

Now, if we could shrink ourselves down to the nano scale and look at them up close, what we'd see is millions of these pancake shaped structures in these orderly little pancake stacks all packed with tiny air bubbles.

When waves of light enter the feather they bounce off of those layers.

Now, when light waves overlap, they can interfere with each other in different ways.

Depending on the wavelength of light, the angle that it enters, the crests and valleys might cancel each other out to dim the color or make it disappear altogether.

But at certain angles, for certain colors of light sometimes those waves line up and are added together to make the reflected color even more vibrant.

All of the light enters but only some light is allowed to come out.

So when you look at the feather from different angles different waves of light line up as they're bounced back to your eye, that is what creates the sensation of shimmering, changing color.

- Yeah, that's the fundamental definition of iridescence is that the color changes depending on the direction that you are looking at it.

- Wow, that's pretty fancy.

Not bad for some little dinosaurs.

Okay, so hummingbirds are cool but they aren't the iridescent royalty of the animal kingdom.

That title probably belongs to beetles.

Biologist J.B.S.

Haldane once said that if nature did in fact have a creator, he has an inordinate fondness for beetles because beetles make up a quarter of all known animal species.

And beetles themselves seem to have a particular fondness for iridescence.

Not every beetle is iridescent, but the thousands that are they have some of the most unbelievable colors in nature.

I mean, honestly, if you didn't know that some of these were real, you'd be forgiven for thinking that they were painted by an artist or a YouTuber trying to trick you.

But they are real.

The outer layer of a beetle's body is made of this super stiff polymer called Chitin.

And when light hits these layers it bends through a process called refraction.

Just like when we look through a glass of water, the light waves seem to bend and not quite line up.

Same thing happens to light in this beetles outer shell.

If those layers are spaced out just right, we're talking a couple hundred billionth of a meter apart, certain colors of reflected light waves will interfere and only certain colors of light escape at certain angles.

Sometimes those refraction reflectors are in the farthest outer layer of the beetle's body or they can be buried a little bit deeper inside.

That's what creates the huge range of iridescence that we see in beetles.

For instance, this one looks like a greenish reddish rainbow, but this one here, you hold it to light, it looks like a hologram.

But being shiny and iridescent may look cool, but one of the most important questions we have to ask in biology, is why something is the way it is.

Turns out these flashy suits of armor may have some surprising functions.

- You're probably thinking, "Oh, how could this possibly be useful as a defense or camouflage or something?

We're pretty sure that nature doesn't bring about any kind of a change that doesn't have a purpose.

And in most cases, you know, the really bright metallic greens are living places with lush green forests with a lot of residual water.

So being shiny and reflective in just the right habitat, in just the right ecosystem, can actually be beneficial.

So the same beetle, if you are six feet from it may be really visible, but if you move back just ten feet, it will start to fade into the background because of the way that the light is playing with it and the position you're observing it.

It's also easy for us to sort of look at this from the standpoint of human color vision which is actually pretty good, versus say, birds, which are the most common predators of insects, including beetles.

And so their perception of what that looks like may be quite different than what our perception is.

There's also lots of other things that being shiny might actually help you find mates.

Depending on the color patterns indicate that you know, maybe, maybe you don't taste very good so a bird would leave you alone.

And in some cases it may actually be about a non-visual thing altogether.

Something like thermal regulation is super important in insects because they can't control their temperature.

They are impacted by the temperature around them.

So having an ability to reflect some of that UV back so that you don't overheat is probably a good thing.

So there's lots of speculated reasons but we still don't have definitive answers as to for one particular species of beetle why it's this way versus another species.

But it obviously serves them or it would've dropped out of the population.

- Now, what I think is the coolest thing about iridescence is how completely distantly related animals can sort of stumble on the same physics for making color.

This is a piece of abalone seashell that I keep on my desk.

It's made of layers and layers of a different hard material called nacre.

Light is bending and reflecting and interfering in almost the same way as in the beetle's shell using a totally different material.

And since the seashell stuff is basically rock the iridescence can even be seen after these shells fossilize.

This is a fossilized ammonite, and it's at least tens of millions of years old, and it's still iridescent.

That is incredible.

But I've been saving my favorite kind of iridescence for last, the beetle that I'm about to show you, I have to admit, I didn't believe it was real, but it is.

Come closer, it's really small.

This is an actual earthling.

It's a type of beetle called a weevil and this particular one looks like it was dipped in glitter but this is also a form of iridescence.

But to understand how this kind of iridescence works we're gonna have to go to an unexpected place.

The rare minerals vault at the Smithsonian in Washington DC.

The rocks and crystals inside of this vault are some of the rarest and most priceless minerals on earth.

To get in, we had to go through an armored door with an actual laser palm scanner like something out of a spy movie, which I was not allowed to film because of security reasons.

- So this is an opal from Australia which is where a lot of the great opals come from.

This is actually a whale bone from Australia that in fact has been opalized.

- Every part of what you just said is ridiculous.

It's a whale bone from Australia that's - I know.

- in the earth that is not just a bone anymore but it's been turned into opal.

- Yeah, and basically the bone was there.

It gets saturated with water that has silica in it.

That porous material that was the bone got filled in with little spheres, opal, basically.

- That's probably the coolest fossil I've ever seen.

- Isn't that pretty?

I mean, one of the most beautiful, right?

- Yeah, that is incredible.

- It is.

Every day I walk in here and I go, something about the earth just amaze me in a different way.

This is one of the most amazing opals I've ever seen in my life.

- This grew in the earth?

- This grew in the earth.

Have you ever seen an opal like that before?

- [Joe] No, I mean, it's coming from every direction - - [Jeffrey] It is.

- and they're big chunks.

- You'd swear it's like somebody put a little battery inside of there, right?

- Yeah.

- I mean, isn't that the most amazing?

- It's like an LED toy.

- [Jeffrey] Look at this one.

This is also (indistinct).

- [Joe] This doesn't look real.

- So you've got, opals are made up of these little spheres of silica.

These are spheres of silica, silicon and oxygen.

They're probably formed out of a silica rich solution.

These little spheres of silica are, you know, very tiny.

- Wow.

- Point five microns, are stacked together very perfectly and it's kind of like, you know, we always stack oranges in a grocery store, right?

- Yeah.

- They're just all stacked together.

And so when the light hits those, they go hurrah, and you get these great flashes of color that we love in opals.

- We are amazingly orderly.

Like it looks like somebody went in with little tiny tweezers and was placing these one by one.

- Basically these little spaces in here, these are about the right separation size to cause diffraction to take place and you get the flashes of color.

Well, light comes in but what happens is light gets reflected off of different layers.

And then, in some places, as the light combines, it combines constructively and other places destructively.

- So some colors are taken out, some are accentuate, - That's right.

- Okay.

- And as you change the angle, that can change.

- Light coming in at a different angles different colors come off at different angles.

- As you're getting different parts of it, you know, the orientation, hits your eye.

I mean, those flashes are just unreal, aren't they?

Isn't that... - That is incredible.

- Yeah.

- The ordered structures that cause iridescence in opals are called photonic crystals, a kind of crystal where the bending and reflecting of light is happening in three dimensions, in a periodic or repeated pattern.

Photonic crystals are also what make these weevils look like they were dipped in glitter.

They're like walking opals.

There are nano-sized three-dimensional repeating structures in the beetle's outer shell.

Some of them are built like honeycombs with orderly spaced pockets of air.

Others are sort of the opposite of that with stacks of evenly spaced spheres with air in between.

And just like in an opal, light is bent, waves combine constructively or destructively and different flashes of color appear.

All of these iridescent colors are the result of physical structures that bend light, not pigments or fluorescence, or any of the other ways that color is created in nature.

Just the strange and beautiful result of light waves interfering with each other.

What I love most about these things isn't that they're beautiful, it's that we can look at rocks and shells and bubbles, and birds and even beetles, and despite how different they all are they're all tied together by this colorful bit of physics.

Kind of makes the universe feel a bit less and a bit more mysterious.

Stay curious.

Welcome to our show, ladies and gentlemen, The Beatles.

I mean the beetles.

All right, turn that away for me.

I can't look at myself.