Why are wet beach pebbles prettier than dry ones?

You know how it is: You pick up a brightly-colored pebble on a wet beach and put the damp rock in your pocket, but when you pull the pebble out of your pocket when you get home, it’s faded and dull-colored. Or you may have watched a sneaker wave rush high on a dry cobble beach, darkening the surf-worn rocks.

What’s going on?

Beach rocks are worn down and smoothed off by rolling in the surf. As they clack and rub against each other and against grains of sand, tiny flecks are chipped off the surface. Ultimately, the chipping rounds off the shape; in the meantime, the chipping makes microscopic dents in the surface that eventually gives the rock a smooth, almost velvety texture. (When very hard beach rocks are polished, the superfine grit and water wear down the edges of the dents to make a much smoother surface.)

Of course, how something looks is a function of our perception of the light that strikes it and bounces towards our eye. According to http://www.askamathematician.com/ (expanded to include a physicist), some light striking the surface of rock reflects off the surface immediately and some light penetrates the rock surface just the tiniest bit before leaving. The light that makes it inside bounces around between entering and leaving, heating the rock’s surface, and refracting on the way out.

When we look at a beach rock, our eyes take in light from both those major paths. We see the color of the rock from the light that’s bounced around just inside because the various minerals absorb and refract the sunlight variously, often “letting out” more of one wavelength.

White light that reflects off the surface of a dry beach pebble is scattered in many directions as it glances off the microscopic landscape of dents. This random scattering is what we notice most, making the dry pebble look hazy.

Adding water changes things considerably: the water fills in the chipped dents while forming a flat outer surface. Now the light bouncing off the (flat) surface of the water tends to reflect in one direction — which is why a wet rock looks shiny. With the surface-bouncing light going in one direction, we see more of the light carrying the color from just under the surface of the rock. Additionally, the skin of water helps light get through the surface of the rock, enhancing the color.

Polishing a rock makes it shiny and colorful because the smoother rock surface directs the surface-bouncing light — which interferes less with the bouncing-around-inside light. Only very hard rocks with tiny crystals polish well because softer or larger-crystal rocks just keep chipping instead of holding their shape while the tiny dents are smoothed out.

Want to keep the pretty colors of a wet beach rock but don’t have a rock polisher? Keep the rocks in a clear jar or a tray of water; if you don’t want to replenish the water (or wash off the algae periodically), you can keep your pretty beach rocks in mineral oil.

Go ahead and put that rock in your pocket! You can restore its sea-side beauty.

Watch the video of the rocks in the photo drying on the Wavecrest Discoveries’ Facebook page: www.facebook.com/wavecrestdiscoveries. For information on how you can arrange an exploration of our fascinating natural history, contact Marty at 541/267-4027, mgiles@wavecrestdiscoveries.com.

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