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Rock color is the first property we pay attention to. Even if the rock is heterogeneous, we are still able to determine the general color of the rock in front of us. Rocks occur in a branch of colors, hues, and saturation. Let us shed a light on the reason why rocks have different colors and what useful information we can get from the color observed.
Rock colors are determined by the minerals that make up the rock. Minerals get their color from the different chemical elements they are composed of. Generally, red-colored rock has a lot of iron in its composition. Light-colored rocks are mostly made of light minerals like quartz, plagioclase, or calcite.
The rock color cannot be explained by minerals only, as there are a lot of other factors besides minerals. They are a rock-forming environment and amount of oxygen during deposition mostly. Stay tuned to explore the factors, which influence one of the most obvious properties of rocks.
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Why Do Rocks Have Different Colors?
Rocks occur in a variety of colors. Rocks can be snow-white like Italian Carrara marble. Rocks can be totally black like gabbro. Rocks can be bright orange-red like bauxite.
What is the reason for the different colors of rocks? Let’s discover how mineral composition and rock-forming environment provide a rainbow pallet of colors.
The rocks have different colors because of different mineral compositions. Light-colored, white, or yellow rocks can be composed mostly of quartz, plagioclase, or calcite. Dark-colored, black, rocks can be made of pyroxene, biotite, or chromite. The red color of the rock is explained by iron oxides.
All rocks are made of minerals. Depending on the number of minerals, rocks are subdivided into:
- monomineralic, which means composed of one mineral only, e.g. quartzite is fully made of quartz;
- and polymineralic, like granite, which is made of quartz, K-feldspar, and plagioclase.
Light colors of these minerals produce the white, grayish, or yellowish, or light-pink color of this rock.
Here is an example of when a slight fluctuation in mineral color can change the color of the whole rock. If granite contains a lot of potassium feldspar it will likely be salmon pink.
Mica or amphibole admixture will result in a grayish or slightly green color.
Dark-colored rocks can be also subdivided according to this scheme. Anorthosite is a type of intrusive igneous rock composed predominantly of calcium-rich plagioclase feldspar. It occurs in light-gray to dark-gray almost black colors.
An example of polymineralic dark-colored rock is peridotite – an ultramafic igneous rock composed of pyroxene and olivine. The color of rock also varies from dark gray to black but with green hues added by olivine.
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- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
What Determines the Color of a Rock?
The chemical composition of rocks usually determines their colors. Chemical elements are organized into highly ordered crystal structures creating a mineral. Different minerals grouping together make up the rock.
The chemical composition of minerals and the mineral combination, which make up the rock, determine its color. Rocks that contain iron can have red hues. Black rocks usually have elevated concentrations of magnesium and titanium. Light-colored rocks are mainly composed of silicon and aluminum.
As the rock inherits the color of minerals, here are the major factors responsible for minerals’ color:
- The presence of an element essential to the mineral composition
- The presence of a minor chemical impurity
- Physical defects in the crystal structure
- The mechanical mixture of very fine impurities
- The presence of finely-spaced structures in the mineral.
Minerals, which are mostly composed of the elements like aluminum, sodium, and potassium (plagioclase, feldspar), are usually colorless or very lightly colored.
As a result, rocks, which are made of these minerals, are white, light-gray, light-yellow, or light-pink colors. Examples of such rocks are granites, syenites, anorthosites).
Inclusions of one mineral within a host rock can also cause a color difference. For example, chlorite inclusions in quartzite, cause the otherwise white-colored rock to be green.
Here are some examples of how different chemical elements’ impurities can change the color of chalcedony:
|Chemical elements and compounds||Color|
|Fe2O3 − TiO2||claret red-dark orange|
|Fe2O3 − Ni||orange to claret red,|
|Cr − Ni − Co||light orange|
|Fe2O3 − Cu||claret red to orange|
|As − Zr and Cr2O3 − MgO||blue color|
If a rock has some secondary copper minerals in its composition, there will be observed some bright blue or green color. Most secondary uranium minerals exhibit bright neon yellow or green colors
To make some generalizations:
- iron is usually responsible for dark red or brown colors,
- manganese and cobalt − for pink,
- chromium − for deep green,
- copper − for blue or green,
- uranium oxides − for yellow and green colors.
TIP: Beaches are usually full of beautiful rocks of different colors. Find out what the most common rocks on the beaches are in the article below:
25 Most Common Rocks on the Beach: How & Where to Find Them?
Which Property of a Rock Determines its Color?
By observing the rock’s color, we can make a very general assumption regarding the weight or specific gravity of the rock.
The specific gravity of the rock can be assumed by the rock’s color. Light-color minerals, which make up light rock, are usually composed of light elements like silicon and aluminum. Dark-colored rocks have greater specific gravity because of heavier minerals and heavier chemical elements, notably iron, and magnesium.
Generally speaking, mineral color points out the specific gravity of the mineral, as minerals that are lighter in color tend to weigh less. Logically, the light-colored rocks will weigh less than dark-colored ones.
Darker minerals typically tend to contain more relatively heavy elements, notably iron, magnesium, and calcium. Grouped together, they make up a considerably heavier rock.
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How Do Rocks Get Their Color?
As the rock color is the most obvious characteristic of rock, it is one of the hardest to interpret. First of all, there is no one general rule for all rocks.
Some similar assumptions can be made about each of the three rock groups, which are igneous, sedimentary, and metamorphic.
Sedimentary rock color can be partly explained by the sedimentation environment during rock formation. The absence of oxygen and deep water deposition results in black color, while terrestrial highly oxidized environments result in red colors. The average indicators result in gray and green colors.
The next picture will shed a light on how the color of sedimentary rock depends on the sedimentation environment:
- Dark gray to black colors means anoxic conditions, which may mean deep water. On the other hand, it could also be a swamp environment.
- Green colors mean an environment with low oxygen concentration, often associated with marine environments.
- Red colors mean well-oxygenated environments, such as river channels, some flood plains, and very shallow marine.
The gray and black colors of rock can be also explained by partially decayed organic matter. Other rock colors are mostly the result of iron rusting.
Ferric iron (Fe+3) produces red, purple, and yellow colors (from minerals like hematite and limonite). Ferrous iron (Fe+2) produces greenish colors.
Speaking about igneous rocks, it’s vital to remember they are formed at high temperatures as a result of the solidification of molten rock and usually at great depths. they can be both light-colored and black-colored, however, tend not to be red because the iron minerals cannot oxidize under these conditions.
Metamorphic rocks usually inherit the color of previously existing igneous or sedimentary rock, but all the rules above cannot be applied to this particular type of rock.
TIP: Quartz crystals can be of different colors. And the color has an impact on their value. Check out how valuable quartz crystals can be in the article below:
How Much Is Quartz Worth? Value for Common Quartz Varieties
Rocks can occur in various colors. Color depends on the mineral composition and oxidizing conditions of the rock-forming environment. To wrap up, the next general statements regarding the rock color derive:
- Rocks made of light-colored minerals like quartz, potassium feldspar, plagioclase, or calcite will be white, light gray, and light yellow.
- At the same time, light-colored rocks have lower specific gravity than dark-colored ones because the minerals they are made of are composed of light chemical elements (silicon, aluminum, sodium).
- Vice versa, dark-colored rocks are made of heavy minerals and heavy elements, in particular iron, magnesium, and calcium. Dark-colored rocks tend to have higher specific gravity than light-colored ones.
Some chemical elements influence rock color more than others. The presence of iron usually results in red, orange-red, and brown color of rocks.
The rock-forming environment also influences the rock color:
- Rocks deposited in deep water, such as in the ocean or deep lakes, are less oxidized, and these rocks tend to be black or gray.
- If rocks are deposited in shallow water, they may be more greenish or brown.
- Rocks deposited on land instead of underwater are more oxidized, especially if they were deposited in wet environments, and so tend to be reddish or tan.
Igneous rocks can be both light-colored and dark-colored because of the minerals they are composed of.
However, igneous rocks are hardly ever rusty-red because the iron minerals cannot oxidize under conditions of high temperatures and great depths.
TIP: Is your backpack big enough to hold all the beautiful colored rocks? If not, check out the best backpacks for rockhounding in the article below:
4 Best Backpacks for Rockhounding: Honest Review & Tips