Imagine you're floating face down in a lake.
No, no, no... not like that! This is NOT the beginning of a horror movie.
Let's try this again. It's summer in the mountains. You've spent the morning hiking, and now it's time for a lazy relaxing afternoon. You're staring at the beautiful multicolored rocks on the bottom the clear lake from the side of your raft.
Suddenly, you begin overthinking your surroundings, and wonder, "Why do rocks look so pretty underwater, but when I bring them home they all get dull and blah?"
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| Wet Rocks in Montana |
It's all about light.
Rocks are Tiny Bumpy Worlds
The surface of a dry rock, even a relatively smooth weathered one, is rough and filled with microscopic bumps, cracks, and pores. When light hits a dry rock, those tiny irregularities scatter the light in all directions.
That scattered light can make the surface look dull, pale, or chalky.
But bring on the water!
When rocks are submerged in the lake, or it starts to rain, or you bust out the rock washing bin from the garage, the water fills in some of those tiny surface spaces and creates a smoother optical surface. Instead of light bouncing randomly off every little microscopic bump, it interacts with the surface differently.
Suddenly a mediocre stone is red, orange, green, purple, beautifully patterned, and possibly the greatest rock you've ever seen.
Water Changes the Way Light Travels
Stick with me here, we might just get a little nerdy.
Light behaves differently depending on what material it's traveling through. When light moves from one material into another, such as from air into water or water into a mineral, its speed changes and its path can bend slightly. This is called refraction.
It's like when you look at a straw in a glass of water, and it looks bent.
Something similar is happening at the surface of a wet rock.
Here's why.
Nearly every material that lets light pass through it has an optical property called a refractive index, which is a number that describes how fast light travels through that thing compared to how fast it travels in a vacuum. A higher number means the material is optically denser, causing light to slow down and bend more sharply.
Still with me here?
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| Partially wet rocks |
The refractive index of our air is around 1.003, which means light is just slightly slower moving through air than in a vacuum. The refractive index of water is 1.333, which means the visible light we can see slows down and bends more noticeably.
Each mineral has a different refractive index, but most rocks are are even higher based on the minerals they're made of, from 1.5 (quartz) to 2.5 (some bright red iron minerals) or more.
But Here's the Really Important Part
To recap, the higher the number, the slower light moves and the more it bends. This changes how much light reflects off the surface and how much can enter the rock. When light moves between materials with different refractive indices, its speed changes and its direction can change. Again, refraction.
The biggest visual difference isn't simply that the water or rocks bend the light.
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| The rock washing bin |
It's that water changes the optical boundary at the surface.
Think about the difference between air and a mineral compared with water and a mineral.
Air and many minerals have a relatively large difference in refractive index. Water is optically closer to many common minerals. That means the transition from water to the mineral is less extreme than the transition from air to the mineral.
When the difference between the materials is reduced, less light is strongly reflected directly from the surface. Being water is kind of like being the middle ground between your wife and your parents... or that one customer and your boss... to uh, help them get along better, you know?
More light can enter the rock rather than being immediately scattered back toward your eyes.
And that's where things get interesting.
Rocks look darker when wet, right?
They appear darker, or more saturated, because more light can travel into the surface of the rock, where dry, rough surfaces scatters a lot of light back toward you. A wet surface tends to scatter less of that light randomly.
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| Wet rock = darker rock, right? |
The result can be a deeper, richer-looking color.
Think about the difference between a dry piece of wood and that same piece of wood after a rainstorm. The wet surface often looks darker and more saturated.
Rocks can do the same thing.
Iron-rich minerals can suddenly show stronger reds and browns. Copper minerals can reveal gorgeous blue and green colors. Jasper can go from “meh” to “WHOA” in about three seconds.
The water didn't make the rock more colorful.
It helped you see the color that was already there.
What About the Shine?
The darker color isn't the only thing water changes. Wet rocks often look much shinier, too.
A thin layer of water smooths over many of those microscopic irregularities. Instead of light bouncing around randomly, more of it can reflect in a more organized way.
It's the same general reason a puddle can reflect the sky or a polished mineral can have a nice glassy shine. The fancy scientific term for this is specular reflection.
A wet rock isn't actually polished, of course. The water is temporarily creating a smoother optical surface, while polishing rocks in a tumbler is actually making the surface smoother. (Bonus! The science of how polishing rocks works explained in a sentence!)
As soon as the water evaporates, the microscopic bumps and pores are exposed again, the light scatters differently, and the shine disappears.
So if you've ever thought a rock looked suspiciously like it had been polished while sitting in a creek, congratulations.
You have been fooled by physics.
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| These rocks are getting polished |
Try It Yourself
Next time you're out rockhounding, find a rock that looks interesting but not spectacular.
Look at it dry.
Then wet it.
Pay attention to what changes. Does the color become stronger? Can you see patterns you missed before? Does the surface look smoother or shinier?
Then let it dry and watch the magic disappear.
Excellent.
You've just done a tiny experiment in geology and optics.
And if you decide the rock looks better wet and therefore needs to come home with you...
Well, that's between you and the rock.
Want to experiment with your own rocks?
- Small spray bottle
- Loupe
- My Favorite Rock and Mineral Field Guide Book
- Weatherproof field notebook
- Rock hammer
Just looking for cool rocks you can't usually find in your location, or fun rockhounding apparel and gifts? Visit the Grumbly Tumbleweed Rock Shop!
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