Thanks to Brilliant for supporting  this episode of SciShow.

Go to Brilliant.org/SciShow  to learn how you can take your STEM skills to the next level! [♪ INTRO]. Coming across this rock looking  back at you might be one of the creepiest experiences a rockhound can have!

It’s called an eye agate, for obvious  reasons, but how do they get so eyeball-y? Rocks or mineral crystals can often remind  us of other things in our daily life. For example, the Desert rose is a  crystal-sand formation that resembles flowers, and the cat’s eye is a mineral called chrysoberyl that reflects light in a  particularly feline-esque way.

And this is probably mostly because  humans just look for patterns everywhere. But something just feels  different about eye agates. When you see something that reminds  you of a human characteristic, you might do some good ol’ anthropomorphism.

These rocks end up looking more  like creepy, disembodied eyeballs. Agates form when fluid rich in silica  flows into gaps between rocks and crystallizes in the empty space as chalcedony. This material is composed of  smaller, sometimes even microscopic, mineral crystals, like quartz.

Now the fluid isn’t magma or anything extreme. Agates form from fluids between  20 and 230 degrees celsius, long after the surrounding rock has cooled. This can happen in many types of rock,  but they’re most common in volcanic rocks.

They’re often rich in silica,  and the leftover heat is perfect for making the hot silica-rich fluid needed! As the fluid flows through the gaps in the rock, the crystals slowly form from  the outside in, layer by layer. Kind of like tree rings if they  formed from the opposite direction!

As the minerals crystalize,  characteristic bands form as the fluid changes slightly in  temperature or composition. Bands of different colors are due  to a combination of their crystal microstructure and include  pigments like iron and manganese. Agates come in all shapes and  sizes, but you need a spherical void in a rock to get one  that looks like an eyeball.

Gas bubbles in lava called  vesicles work perfectly for this. The uncanny appearance of these rocks  has led to them being marketed and sold as evil eyes or third eyes, and claims of supernatural powers often come along with them. And while they clearly have spectacular  natural beauty, most that you see for sale in gift shops have likely had a bit of human help.

For example, the way that they’re  polished can do a lot to help make an association with an actual eye. Even though they can have vibrant  natural colors, they’re usually dull. But because their crystal structure is  quite porous, they can easily be dyed.

By exposing an agate to various  chemicals, you can turn it almost any color you want! So the next time you’re looking for  cool rocks, keep an eye out for agates. They may not look like much at  first, but with a bit of help, they can be beautiful or spooky!

A place where you can find more great  science is with today’s sponsor, Brilliant! Brilliant is a website and app built off  the principle of active problem solving: you learn best while doing  and solving in real-time. And they’ve overhauled their  courses to be even more interactive.

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