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Rock colour is the first property we pay attention to. Even if the rock is heterogeneous, we can still determine the general colour of the rock in front of us. Rocks occur in a branch of colours, hues, and saturation. Let us show why rocks have different colours and what useful information we can get from the observed colour.
Rock colours are determined by the minerals that make up the rock. Minerals get their colour from the different chemical elements they are composed of. Generally, red-coloured rock has a lot of iron in its composition. Light-colored rocks are mostly made of light minerals like quartz, plagioclase, or calcite.
However, the colour of a rock is not only dictated by its mineral composition. Other factors, such as the rock-forming environment and oxygen during deposition, also play crucial roles. By exploring these factors, we can understand one of the most conspicuous properties of rocks and unravel the stories they hold within their colourful surfaces.
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Why Do Rocks Have Different Colors?
Rocks occur in a variety of colours. Rocks can be snow-white like Italian Carrara marble. Rocks can be black, like gabbro. Rocks can be bright orange-red like bauxite.
What is the reason for the different colours of rocks? Let’s discover how mineral composition and rock-forming environment provide a rainbow pallet of colours.
The rocks have different colours because of different mineral compositions. Light-colored, white, or yellow rocks can be composed mostly of quartz, plagioclase, or calcite. Dark-coloured, black rocks can be made of pyroxene, biotite, or chromite. Iron oxides explain the red colour of the rock.
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 colours of these minerals produce this rock’s white, greyish, yellowish, or light-pink colour.
Here is an example of when a slight fluctuation in mineral colour can change the colour 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 greyish or slightly green colour.
Dark-coloured rocks can also be subdivided according to this scheme. Anorthosite is an intrusive igneous rock composed predominantly of calcium-rich plagioclase feldspar. It occurs in light-grey to dark-grey, almost black colours.
An example of polymineralic dark-coloured rock is peridotite – an ultramafic igneous rock composed of pyroxene and olivine. The colour of rock also varies from dark grey to black but with green hues added by olivine.
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What Determines the Color of a Rock?
The chemical composition of rocks usually determines their colours. Chemical elements are organized into highly ordered crystal structures, creating a mineral. Different minerals grouping make up the rock.
The chemical composition of minerals and the mineral combination that makes up the rock determine its colour. 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 colour of minerals, here are the major factors responsible for minerals’ colour:
- 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, mostly composed of aluminum, sodium, and potassium (plagioclase, feldspar), are usually colourless or lightly coloured.
As a result, rocks made of these minerals are white, light grey, light yellow, or light colours. Examples of such rocks are granites, syenites, and anorthosites).
Inclusions of one mineral within a host rock can also cause a colour difference. For example, chlorite inclusions in quartzite cause the otherwise white-coloured rock to be green.
Here are some examples of how different chemical elements’ impurities can change the colour of chalcedony:
| Chemical elements and compounds | Cr − Ni-Co |
|---|---|
| Fe2O3 | claret red |
| Fe2O3 − TiO2 | claret red-dark orange |
| Fe2O3 − Ni | orange to claret red, |
| Cr − Ni − Co | light orange |
| As | yellow |
| Fe2O3 − Cu | claret red to orange |
| As − Zr and Cr2O3 − MgO | blue color |
If a rock has some secondary copper minerals, some bright blue or green colour will be observed. Most secondary uranium minerals exhibit bright neon yellow or green colours
To make some generalizations:
- Iron is usually responsible for dark red or brown colours,
- manganese and cobalt − for pink,
- chromium − for deep green,
- copper − for blue or green,
- uranium oxides − for yellow and green colours.
TIP: Beaches are usually full of beautiful rocks of different colours. 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 colour, we can make a general assumption regarding the weight or specific gravity of the rock.
The rock’s colour can assume the specific gravity of the rock. Light-colour minerals comprise light rock and usually contain light elements like silicon and aluminium. Dark-coloured rocks have greater specific gravity because of heavier minerals and chemical elements, notably iron and magnesium.
Generally speaking, mineral colour indicates the mineral’s specific gravity, as lighter minerals tend to weigh less. Logically, the light-coloured rocks will weigh less than the dark-coloured ones.
Darker minerals typically tend to contain more relatively heavy elements, notably iron, magnesium, and calcium. Grouped, they make up a considerably heavier rock.
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How Do Rocks Get Their Color?
As the rock colour 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.
The sedimentation environment can partly explain sedimentary rock colour during rock formation. The absence of oxygen and deep water deposition produces black colour, while terrestrial highly oxidized environments result in red colours. The average indicators result in grey and green colours.
The next picture will shed light on how the colour of sedimentary rock depends on the sedimentation environment:
- Dark grey to black colours mean anoxic conditions, which may mean deep water. On the other hand, it could also be a swamp environment.
- Green colours mean an environment with low oxygen concentration, often associated with marine environments.
- Red colours mean well-oxygenated environments, such as river channels, some flood plains, and very shallow marine.
Rock’s grey and black colours can also be explained by partially decayed organic matter. Other rock colours are mostly the result of iron rusting.
Ferric iron (Fe+3) produces red, purple, and yellow colours (from minerals like hematite and limonite). Ferrous iron (Fe+2) produces greenish colours.
Speaking about igneous rocks, it’s vital to remember they are formed at high temperatures due to the solidification of molten rock and usually at great depths. They can be light and black; however, they tend not to be red because iron minerals cannot oxidize under these conditions.
Metamorphic rocks usually inherit the colour 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 colours. And the colour 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
Conclusion
Rocks can occur in various colours. Colour depends on the mineral composition and oxidizing conditions of the rock-forming environment. To wrap up, the next general statements regarding the rock colour derive:
- Rocks of light-coloured minerals like quartz, potassium feldspar, plagioclase, or calcite will be white, light grey, and light yellow.
- At the same time, light-coloured rocks have lower specific gravity than dark-coloured ones because the minerals they are made of are composed of light chemical elements (silicon, aluminum, sodium).
- Conversely, dark-coloured rocks contain heavy minerals and elements, particularly iron, magnesium, and calcium. Dark-coloured rocks tend to have higher specific gravity than light-coloured ones.
Some chemical elements influence rock colour more than others. Iron usually results in red, orange-red, and brown colour of rocks.
The rock-forming environment also influences the rock colour:
- Rocks deposited in deep water, such as in the ocean or deep lakes, are less oxidized, and these rocks tend to be black or grey.
- 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-coloured and dark-coloured because of the minerals they are composed of.
However, igneous rocks are hardly rusty-red because the iron minerals cannot oxidize under high temperatures and great depths.
TIP: Is your backpack big enough to hold all the beautiful coloured rocks? If not, check out the best backpacks for rockhounding in the article below:
4 Best Backpacks for Rockhounding: Honest Review & Tips