When you’re browsing the shiny stones  in your local museum gift shop,   there’s a few staples you can expect to find.

Pointy white quartz crystals, purple  amethyst, shiny golden pyrite, and this:   layered brown tiger’s eye, winking at you  with its classic cat’s eye reflection. Since these stones are like,   everywhere, you might assume that we know  everything there is to know about them.

But it turns out that for over a century,   we were completely wrong about how tiger’s  eye formed, and even what it’s made of. Here’s how we got there, and the research  that finally helped us crack the case! That is a pun that you’re gonna get in a minute. [♪ INTRO] Tiger’s eye is the name commonly given to this  gorgeously golden brown banded form of quartz.

What really sets it apart from the other rocks on  the shelf is its silky reflection, which creates   a line of light moving across its polished surface  that looks like the narrowed slit of a cat’s eye. It’s a phenomenon known as chatoyancy,   which is similar to the asterism in  things like star emeralds or rubies. The stone is made up of ultra-fine fibers,   all lined up so they catch the light together  and create a single band of reflected light.

That chatoyancy has made tiger’s  eye super popular among collectors,   and many people carry some around for good luck. Including our director Hiroka! But while the mechanics of light  reflection might be well-understood,   the chemistry and geological history of these  gemstones hasn’t always been so straightforward.

Their story begins in the late 1800s,   when samples of tiger’s eye were  incredibly rare and highly prized. At that time, tiger’s eye was reportedly  worth 35 times its weight in gold,   but nobody had done much scholarly  work on like, what it was made of. That was, until Ferdinand Wibel took up the task.

Wibel was a German chemist who  used scientific chemistry to   study ancient materials, so he was  the perfect guy to investigate the   mineralogical mysteries of tiger’s  eye and its blue cousin, hawk’s eye. Wibel noticed that the fibrous texture and  color of hawk’s eye looked very similar to   another mineral, called crocidolite,  also known as ‘blue asbestos’. And if you’re wondering, yes, it is that asbestos.

In nature, crocidolite is blue and fibrous,   just like hawk’s eye, which is  probably where Wibel got the idea. But based on the minerals’ hardnesses  and specific gravities, it was clear   that hawk’s eye was composed of quartz,  despite looking just like crocidolite. So based on all that, Wibel concluded  that hawk’s eye formed when quartz   had replaced existing crocidolite material,   changing the stone’s chemistry while  maintaining its original asbestos texture.

According to Wibel, the blue hawk’s  eye represented a partial replacement,   with residual crocidolite trapped  inside giving the gem its blue color. Tiger’s eye, then, represented a full replacement,   with the brown color created as the  original mineral was broken down. On the face of it, this was a pretty  sensible idea, since this kind of thing,   known as pseudomorphism, happens  pretty frequently in geology.

The most obvious example is fossils. When you’re looking at an ancient dinosaur  bone, most of the time what you’re really   seeing is a rock that replaced all the  living-bone bits with hardened minerals. This is how we get ammonites made of pyrite  and petrified wood, which is mostly silica.

But pseudomorphism isn’t just for fossils. We spot them in rocks when the shape of a crystal   doesn’t match up with what you’d  expect from its chemical make-up. For instance, pyrite can pseudomorph into the  iron oxide minerals limonite and goethite,   making cube-shaped crystals that are composed  of minerals that don’t usually like to be cubes.

So Wibel’s pseudomorphic origin  for hawk’s eye and tiger’s eye,   published in 1873, made  tangible and theoretical sense. It was accepted by the scientific community and   repeated in mineralogical guides and  textbooks for more than 125 years. Thing is, nobody ever actually  checked if it was true.

Wibel gets a pass since he wasn’t working with  a lot of high-tech tools, but at no point in   the decades that we’ve had chemical probes or  superpowered microscopes did anyone think “hey,   should we actually look at whether this  hypothesis from the 1800s is true or not?” Until 2003, when researchers from Penn State   used modern high-res imaging techniques to  finally take a closer look at tiger’s eye. And they found that one of Wibel’s core  assumptions had been flat out wrong. The quartz in the tiger’s eye bands wasn’t fibrous  at all, but laid out in chunky elongated columns.

The crystals were up to 1 millimeter across  and 10 millimeters long, and while that’s not   exactly massive, that’s still way thicker  than the asbestos fibers of crocidolite. So it didn’t make sense that these were a  search-replace pseudomorph for crocidolite,   and those quartz crystals grew  that way all on their own. And yet, weirdly, they did still find  some inclusions of that crocidolite.

Hawk’s eye had more, and tiger’s eye had less. The inclusions occurred in sequences of short  fibers around a hundred millimeters long,   which were often, but not always, oriented  parallel to the lengths of the quartz columns. And when those two directions differed,  the tiger’s eye chatoyancy reflection was   at right angles to the fibers, not  the quartz crystals, so they could   tell it really was the crocidolite that  caused the silky reflection, after all.

So what’s going on? The 2003 researchers proposed that  tiger’s eye formed under what they   call a ‘crack-seal’ process in areas  affected by large-scale tectonic forces. That’s the pun from the beginning!

You start with a pre-existing rock  that contains some crocidolite. That rock is contorted by massive  tectonic stress, then it cracks,   creating a gap that’s then filled by  quartz-rich fluids circulating deep underground. Quartz crystals grow into the  cracks and create thin quartz veins,   and any crocidolite fibers that had bordered  those cracks grow into the empty spaces.

As a result, you get quartz crystals  surrounding the fibrous inclusions. But tectonic forces aren’t often  a one-and-done kind of thing. The stress continues and the vein cracks  again, allowing more quartz and crocidolite   to grow into the space, with quartz  trapping the thin asbestos fibers.

This happens repeatedly and  the cracks grow incrementally,   as little as a tenth of a millimeter at a time. It can take 100 separate cracking events to build  up a one centimeter-thick vein of tiger’s eye. Within this vein, the crocidolite inclusions  form a trail, anchored into the original rock,   that preserve a history of the cracking events.

If the veins open up straight, then the  inclusions stay aligned with the quartz. But if there’s any sideways motion,   then the asbestos fibers end up  in a sort of staircase shape. Through high-resolution microscopy, these  textural relationships have been revealed   for the first time, and they fit with  the wider geological setting for where   tiger’s eye is found, in places  like South Africa and Australia.

Both of these regions have experienced a ton of  folding and faulting in their tectonic histories. For instance, South Africa’s Cape  Asbestos Field was involved in two   major mountain building events  around two billion years ago,   which involves a ton of geological pressure,  enough to crack rocks again and again. So in a very real way, the smallest  scale structures in these gemstones   are a cat’s eye reflection of the ancient,  large-scale tectonic events that formed them.

By the way, if all this talk of asbestos has you   side-eyeing the tiger’s eye  on your shelf, don’t stress. Asbestos fibers only cause problems  when they’re breathed in, and those   columns of quartz in there keep the  asbestos fibers fully contained. As long as you aren’t grinding it into  powder, tiger’s eye is totally safe!

Which is great news, because SciShow  Rocks Box subscribers can look forward   to receiving their own piece of  shimmering tiger’s eye this month. Every month, our subscribers receive a hand-picked  mineral or fossil specimen, perfect for display. And, we also offer some of  the fan-favorite minerals   for purchase a la carte, so head over to  Complexly.store/rocks to check it out. [♪ OUTRO]