Some rocks change color when wet because water can reveal pigments that were initially hidden, or dissolve colored minerals present in the rock, altering its natural hue.
The surface of rocks is often made up of tiny pores and cracks. When water is poured onto them, it seeps into these small spaces, replacing the air that was previously there. Air reflects light easily and makes the rock appear lighter to the eye. When this air is replaced by water, the visual effect is no longer the same. The reflection decreases and light penetrates more, revealing a more intense and saturated color of the rock. Thus, deeper shades appear that were not noticeable on the dry rock.
When a rock is dry, the roughness and small irregularities on its surface scatter light in all directions. As a result, a paler or duller color is perceived. However, in a wet state, water forms a thin layer that fills in these small crevices, making the surface smoother and more even. This directly alters the way light strikes the stone: part of it slightly penetrates into the water and the rock, undergoing a phenomenon of refraction (bending of light). The other portion is directly reflected at the surface of the water at a more precise angle, which intensifies the visible colors. The rock then appears darker, its colors become more vibrant and contrasted, revealing previously hidden shades. This visual change is purely optical, with the chemical composition of the rock remaining unchanged.
Each rock is composed of particular minerals, and each absorbs or reflects light differently. Some minerals like quartz or feldspars become more translucent when they are wet. This enhances the shades and sometimes reveals colors that are invisible when dry. Conversely, other minerals remain matte and barely change color when wet. It all depends on their internal structure, density, and natural ability to let light in or reflect it.
Granite is a typical example: light and dull when dry, its colors become bright and contrasting once wet. The same goes for pebbles, often gray and unremarkable when dry, but taking on a much more intense and shiny hue as soon as they are moistened. Chalk, white and matte in the open air, reveals darker and subtler shades when in contact with water. The same thing happens with slate, light gray when dry, which tends to a dark bluish gray when it becomes wet. Water acts a bit like a temporary varnish, revealing the deep and rich tones of these rocks.
Did you know that some fossils are much easier to identify when they are wet? Wetting their surface with water often enhances the contrasts, making it easier to distinguish the fine details of the fossil shapes.
Some rocks, such as pumice, whether wet or dry, float on the surface of the water due to their numerous air-filled cavities, which give them a lower density than that of water.
Did you know that gemstones like opal and labradorite exhibit exceptional iridescence? This characteristic comes from the unique refraction and reflection of light within the minerals themselves and often becomes even more spectacular when they are wet.
Geologists sometimes use the so-called water test method to better identify certain minerals. By wetting an unknown rock, various subtle details, such as mineral veins or hidden textures, can become visible and facilitate its identification.
Yes, generally the color of the rocks returns to normal when they are completely dry. However, some very porous rocks may take longer to regain their original appearance.
The color of a wet rock is useful but rarely sufficient to accurately identify a mineral. It is an additional clue to consider in conjunction with other criteria such as hardness, texture, or chemical reaction.
When a rock is wet, water covers its surface and reduces light scattering. Thus, it reflects more light uniformly towards our eyes, giving the rock a brighter or more luminous appearance.
It is temporary. The altered color of the wet rocks gradually disappears as the water evaporates from their surface.
No, only certain rocks change significantly in color when they are wet. This mainly depends on their porosity, mineralogy, and the optical effects created by water on their surface.
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