The natural world has done a good job of producing  things of pretty much every color you can imagine.

And some things are showy with their color, flashing them all in a rainbow of iridescence  depending on how you look at them. There are iridescent butterflies and  beetles, iridescent peacocks and pigeons.

Oil and soap on water create  iridescent colors, too. There are even some iridescent  gemstones, like opals, and pearls. But sometimes, iridescence can  be found in unexpected places, like on the surface of everyday  iron ore, known as hematite.

And while iridescent hematite is a beautiful  and unusual addition to collectors’ shelves, it’s a bit of a mineralogical puzzle, because  scientists aren’t sure exactly why it’s so pretty. [♪ INTRO] Hematite is an iron oxide mineral, which  is one of the main components of iron ore. With two parts iron to three parts oxygen, it  actually has the same chemical formula as rust, but in its mineral form it’s like  rust has had the ultimate makeover. Hematite can occur as silvery crystals, red  rounded masses looking like petrified bubbles, or as glittery, finely crystalline chunks.

But if hematite is like  rust with a make-up artist, then iridescent hematite, which flashes  its rainbow colors all over its surface, looks more like a toddler left  unattended with the make-up bag. The story of this rainbow-colored  rock begins relatively recently. In the 1990s, an American mineral dealer  by the name of Rock Currier – yes, really – heard about an especially colorful rock coming  out of an iron mine in southeastern Brazil.

Apparently, every year for a  festival in the local village,   the miners would load up a dump  truck with this rainbow-colored rock, and dump it out all over the road, where  they’d sparkle and shine like peacock feathers. So when Rock Currier got there, true to his name, he loaded about 15 tonnes of this “color rock”  into barrels, and shipped them back to the US. This came as a surprise to the locals, who were  much more used to dealing in serious iron ore, and only thought of their  peacock-colored hematite as a curiosity.

Despite his optimism, Rock Currier didn’t  have much luck selling his color rock, even though he was offering a  bargain price of about $6 per kilogram. But once he realised that individual  pieces were valued as collector’s items, and were even being used for jewelry,   he fished the best bits out of  his barrels, and made his fortune. Since then, Currier’ color rock  and other specimens of iridescent   hematite from around the world  have become prized collectibles.

The samples are sometimes also known as turgite,   after the Turginsk mine in Russia where  iridescent hematite was first found. But official mineral names are only  ever given to distinct minerals,   and that’s where turgite runs into trouble. Although we know this is hematite, and we can   see that it’s an especially  colorful variety of hematite, it seems that no one is really sure  what exactly sets this variety apart,   or how its iridescence works.

The first to tackle the problem were a  pair of researchers from Caltech in 2003. They used high-resolution microscopes  and light absorption techniques to look   at the physical and chemical properties  of the iridescent hematite from Brazil. These analyses seemed to show that the iridescence   occurred as a result of a separate thin  layer on top of the hematite crystals.

The layer had a higher concentration  of aluminum and phosphorus,   and the researchers suspected it was a different   mineral than the hematite underneath, but the  layer was too fine to be able to say any more. But even without knowing what it was  made of, the structural information   suggested that the colours were caused by  something called thin film iridescence. This is a similar mechanism  to how oil spilled over water,   or soap bubbles, produce their rainbow sheen.

It happens when a thin layer has a different  refractive index to the underlying material. Light rays pass through the film and are  bent, or refracted, by different amounts. So when the light comes back out, the waves are  slightly out of phase and interfere with each   other, favoring some wavelengths and colors  over others, producing a rainbow effect.

Even though the chemistry of the  layer itself hadn’t been identified,   no progress was made for years, until  in 2018 researchers from Penn State   University published a paper in which  picked up the iridescent hematite again. These researchers noted something that didn’t  fit with the previous thin film explanation. They reported that, whenever they broke a new  piece off a chunk of the Brazilian hematite,   there was more iridescence inside.

The colors seemed to penetrate much  further than just a thin coating. So this time, they threw the whole  toolbox at the Brazilian color rock,   using more advanced instruments than  were available the first time around. A variety of different tools and techniques looked  at the mineral’s surface features and chemical   composition, mapped the surface topography on a  fine scale, and measured the crystal structure.

And the analyses seemed to confirm  their ‘color all the way down’ theory. Instead of a layer on top, the  high-res scans showed that the   hematite itself was made up of tiny  nanocrystals, shaped like cigars. The crystals were made of iron oxide,   but with aluminum and phosphorus mixed  in instead of iron every 10 atoms or so.

They are stacked on top of each  other at 120 degree angles,   creating a threefold symmetry of star-like shapes. And each crystal was just a few hundred  nanometers long, and about 50 nanometers wide. Now those sizes are important, because they’re  within the same range as the wavelength of   visible light, and the researchers think  it’s this that’s causing the iridescence.

Essentially, the nanocrystal lattice  creates what’s called a diffraction grating. That’s where light bouncing off a structure with  regular ridges and grooves interferes with itself,   amplifying some wavelengths and  colors and canceling out others. This so-called structural color creates the  multicolored reflections from CDs and DVDs,   but also the colors of some butterfly wings,  and the rainbow coloring to thin wispy clouds.

In the geological world, it causes the colors  of opals and the shiny blue labradorite. And according to the Penn State  researchers, the iridescent hematite too. But that’s not the end of this story.

In early 2025, the original Caltech scientists   published a new study that doubled down  on their thin-film interference theory. This time, they didn’t just look  at the Brazilian color rock,   but studied examples of iridescent hematite  from Mexico, the US, the UK, and Italy,   using similar high res techniques  to their Penn State colleagues. And their analysis showed that,  in all cases they could find,   the rocks had a thin coating of  the cigar-shaped nanocrystals.

They even showed places where  the star-shaped lattice stopped,   revealing smooth hematite material underneath. Among all the specimens they looked at,   they couldn’t replicate the findings  of nanocrystals all the way down. So which is the right?

Thin film or diffraction grating? Oil slick or DVD? Maybe one or the other sets of researchers  missed something, or assumed too much.

Or it could be both, with different mechanisms   genuinely operating for different  samples, or even within samples. The techniques used to measure nanoscale  crystals are pinpoint accurate,   but they can only ever look at  a tiny area of a single sample. What’s more, it’s getting more and more  difficult to study the Brazilian color   rock, because our rock courier, Rock  Currier’s 15 tonnes have all been sold.

And we can’t just go and  grab more because the seam   of iridescent hematite is sitting under a road. A road that leads to… the rest of the  very important and lucrative iron mine. So digging it up for the sake of a few  sparkly specimens isn’t really an option.

But there are still places where  researchers and enthusiasts can get   their hands on some colorful hematite,  including several locations in the US. One of those places is Graves Mountain in Georgia,  and SciShow rocks box subscribers are in luck,   because that is the source of  this month’s collectors piece. Subscribers will receive their very own piece  of iridescent hematite, just like this one,   and whatever actually causes it to  be shiny, it will be very shiny.

Subscribers receive an ethically-sourced mineral   every month, in this beautifully  designed box, right to their door,   along with a handy info card telling you  all the fun facts about your new specimen. If you want to sign up for the waitlist, head  over to Scishow.rocks or click the link in the   description, where you can also peruse our other  a la carte offerings of all kinds of rocks. [♪ OUTRO]