Why do geologists lick rocks?
― Wetting a rock clears the "white haze" on its surface
People who study rocks in the field will lick a freshly broken rock or splash water from their bottle onto it. It's not bad manners. A dry rock's surface is covered in tiny bumps and grooves that scatter light back as a white "haze." Wetting it clears that haze away, revealing the mineral's true colour and the shape of its grains.
At a riverbank, you find a beautiful stone with red and green stripes, glowing in the water.
You pocket it and take it home. The next day, looking at it on your desk, it's turned into a dull, whitish, nondescript rock.
The colour hasn't actually faded. Dip it in water again, and the pattern comes right back. The reason geologists lick rocks is to recreate that "dipped in water" state on the spot.
There are only two reasons wetting makes it visible
A dry rock's surface is rough with tiny scratches and powder. Each bump reflects light a little, scattering it in all directions. This is the white haze sitting on top of the colour. Water fills in the bumps and flattens the surface.
Light reflects off a surface less when travelling from water into rock than when travelling from air into rock. Light that isn't reflected away enters the rock and comes back out carrying the mineral's colour. That's why the colour looks deeper and sharper.
Both reasons come down to "how much light bounces off the rock's surface." Let's look at the first one.
A dry rock is covered in a white haze
Zoom in on a freshly broken rock, or one that's been weathered by wind and rain, and the surface is covered in tiny bumps and grooves, with fine powder-like fragments clinging to it. When light hits such a surface, a small portion bounces back. Since each tiny facet points in a different direction, the reflected light scatters every which way.
That reflected light hasn't passed through any mineral, so it carries no colour. That means from any viewing angle, a thin layer of white light sits on top of the rock's true colour. That's the "dullness" you see. It's the same reason frosted glass looks whitish.
Look at the left side of Figure 1. On the dry surface, white arrows point every which way. On the right, as in the diagram, a film of water fills in the bumps and the surface becomes flat like a mirror. The reflected light now lines up in a single direction, producing a "glare" only when viewed from that angle. From any other angle, you see straight into the rock with no haze.
Wetting a rock cuts surface reflection to one-eighth
The second reason lies in how light reflects at the "boundary between two different materials." The number describing how much a material slows light down is called the refractive index. The bigger the difference in refractive index across a boundary, the more light bounces off it.
Air has a refractive index of 1.00, while quartz, a common rock-forming mineral, has about 1.54. Because the gap is large, about 4.5% of the light hitting that boundary is reflected. Water, though, has a refractive index of 1.33, which is much closer to quartz. At a water-quartz boundary, only about 0.5% is reflected — about one-eighth as much.
Light that isn't reflected travels deep into the grain. There, the mineral absorbs part of its colour, and the rest returns to your eye. That's why a wet rock looks darker, richer, and more vivid. The boundaries between grains also become easier for light to pass through, so they stand out more clearly.
Try the slider in Figure 2 to change the refractive index of the liquid coating the rock's surface. Moving from air to water to oil — each step closer to quartz — less light is reflected and the haze clears.
Gravestones and stone floors are polished to a glossy smoothness. Polishing, like wetting with water, is a way of "removing the bumps." Gem appraisers sometimes submerge a stone in a liquid with a refractive index close to the gem's, to see internal flaws and patterns. When the indices match closely enough, reflection at the boundary nearly vanishes — the stone's outline can all but disappear.
Among fossil hunters, it's said that bone fossils, riddled with tiny pores, make the tongue feel like it's being sucked onto the surface. That said, some minerals contain substances harmful to the body. For an unfamiliar rock, it's safer to splash water on it rather than lick it.
Summary
A dry rock looks dull not because its colour has faded, but because white light scattered by surface bumps lies on top of it. Wetting fills in the bumps with water, and also makes light reflect less at the boundary with the rock. Light travels deep into the grains and returns carrying the mineral's colour. A geologist's lick is the simplest observation tool you can use on the spot.
A rock's colour doesn't vanish when it dries — it's just hidden behind white light.
Water acts like a lens, clearing that haze away.
For why riverbed stones are round, see "Why are river stones round?", and for why white paper still looks white in a dim room, see "Why does white paper still look white in a dim room?"
- Pick up two dull, whitish pebbles from your garden or a park. Leave one dry, and submerge the other in a glass of water.
- Compare the two side by side. The one in water should look darker in colour, with grains and patterns standing out clearly.
- Take it out and watch it dry over time. You'll see the whitishness creep back in first from the fine edges.
If you have some on hand, try comparing a rock with a drop of salad oil, too. Oil has a refractive index closer to quartz than water does, so the colour may look even sharper.
Want to know more? ― Terms, formulas, and textbook connectionsWe mark clearly which level of schooling each part belongs to, from middle school science to university specialist courses
- MSCovered in middle school science
- HSCovered in high school "Physics" or "Earth Science"
- HS+High school advanced material, or textbook sidebar content
- UnivContent from university specialist courses (optics, petrology) not covered in high school
- ResearchNot yet settled "textbook fact" even at university — an area researchers are actively studying
MSTerms: this phenomenon has a name
- Diffuse reflection: light bouncing off a rough surface in many different directions. This is what produces the white haze on a dry rock.
- Refractive index: a number describing how much a material slows light down. Vacuum is 1, water is 1.33, and quartz is about 1.54.
- Rock-forming minerals: the minerals that make up a rock. In granite, these are quartz, feldspar, biotite mica, and so on.
MSHSChecking with a formula: how much less light reflects off wet quartz than dry quartz?
We calculate the reflected fraction for light hitting a boundary head-on.
| In symbols | R = ( (n₂ − n₁) ÷ (n₂ + n₁) )² |
| In words | Reflected fraction = (difference in refractive index ÷ sum of refractive index) squared |
| Where it comes from | Treating light as an electromagnetic wave and requiring the electric and magnetic fields to match across the boundary gives the "Fresnel equations"; this is the case of light hitting straight-on. |
What the symbols mean: R is reflectance (the fraction of light reflected, no unit), n₁ is the refractive index of the medium the light comes from (air or water), and n₂ is the refractive index of quartz.
| Refractive index of air | 1.00 |
| Refractive index of water | 1.33 |
| Refractive index of quartz (approx.) | 1.54 |
| Dry rock: difference in refractive index | 1.54 − 1.00 = 0.54 |
| Dry rock: sum of refractive index | 1.54 + 1.00 = 2.54 |
| Difference ÷ sum | 0.54 ÷ 2.54 ≈ 0.213 |
| Squared for reflectance | 0.213 × 0.213 ≈ 0.045 |
| Wet rock: difference in refractive index | 1.54 − 1.33 = 0.21 |
| Wet rock: sum of refractive index | 1.54 + 1.33 = 2.87 |
| Difference ÷ sum | 0.21 ÷ 2.87 ≈ 0.073 |
| Squared for reflectance | 0.073 × 0.073 ≈ 0.0053 |
| How many times more does a dry rock reflect? | 0.045 ÷ 0.0053 ≈ 8.5 |
A dry quartz surface reflects about 4.5% of light, while a wet surface reflects about 0.5%. That's a small difference for a single facet, but on a dry rock, countless scratches and powder facets each reflect their own 4.5%, and together they add up to the white haze. Wetting gathers them into a single flat water surface, and also cuts reflection at the rock boundary to about one-eighth.
HSHS+The link between reflection and refraction
HSIn high school physics, you learn the law of refraction (Snell's law), which describes the angle at which light bends at a boundary. The same ratio of refractive indices that determines how much light bends also determines how much reflects. If there's no difference at all, light passes straight through, neither bending nor reflecting.
HS+Even the surface of the water itself (the air-water boundary) reflects about 2% of light. But on a wet rock, that water surface is flat, so this reflection goes in just one direction. Look from any other angle, and you see straight into the rock with no haze. Polarised sunglasses can cut much of this glare from the water surface, too.
Univ"Multiple scattering" — light getting lost among a mass of grains
In materials made of many grains, such as sand or rock, light bounces repeatedly off grain boundaries as it travels — this is called multiple scattering. When the gaps between grains are filled with air, reflection at each boundary is large, and light is quickly bounced back out, giving a whitish appearance. When the gaps are filled with water instead, reflection at each boundary weakens and light travels deeper in. That gives minerals more chances to absorb it, making the whole surface darker and more vivid. This explanation was quantitatively demonstrated in Lekner and Dorf's 1988 paper, "Why some things are darker when wet." The relationship between refractive index and reflection is expressed by the Fresnel equations, split by polarisation.
📖 For the derivation and further reading: Fresnel equations (Japanese Wikipedia) / Lekner & Dorf (1988) Why some things are darker when wet
ResearchWhat's still not fully understood
- The relationship between moisture and appearance How much water is needed to darken slightly damp soil or sand by how much varies greatly with grain size and shape, and no unified formula has been established yet.
- Reading soil moisture from satellites Methods for estimating soil water content from ground brightness are being researched, but separating out the effects of soil type and vegetation remains difficult.
- The colour of Martian rocks Rovers scrape or brush the surface of rocks to see the colour underneath. How to correct for the effect of dry dust is still a work in progress.
In other words, this article too describes things "as currently understood." The fraction of light reflected at a surface is fixed by a reliable formula, but how a whole mass of grains appears is still an active area of research.
Connections to the curriculum (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | 1st-year science, "Reflection and refraction of light," "Formation of igneous rock" | White haze from diffuse reflection, granite grains |
| HS | Physics "Refraction of light," Earth Science "Rocks and minerals" | Relationship between refractive-index difference and reflection |
| HS+ | Physics (advanced) "Polarisation" | Glare off a water surface and polarised sunglasses |
| Univ | Optics (Fresnel equations), petrology | Multiple scattering and the darkness of wet materials |
| Research | Remote sensing, planetary geology | Estimating soil moisture, observing Martian rocks |
| ― | Everyday connections | Riverbed stones, polished gravestones, roads that look darker in the rain |
- Lekner, J. & Dorf, M. C. (1988) Why some things are darker when wet. Applied Optics 27, 1278.
- フレネルの式 (Fresnel equations) - Japanese Wikipedia
- 屈折率 (Refractive index) - Japanese Wikipedia
- Hecht, E. Optics (reflection and refraction of light, Fresnel equations)
※ This article is a general-audience science explainer. The figures given are approximate, meant to help convey the underlying mechanism. Refractive index varies slightly by mineral type and the colour of light. If you examine rocks outdoors, be sure to get permission for entering the site and watch your footing.