Why Do Rocks Have Different Colors?
Rockhounds know, better than anyone else, that different minerals have different colors. Some stones even have outstanding patterns in them, with bright colors seemingly distributed at random throughout the piece. All of this has to come from somewhere, right?
Right indeed, let’s get down to it and we’ll help you understand why rocks have different colors, and even where some of those colors come from!
Mineral Base and Impurities
Minerals have to be a single color if they’re 100% pure. It’s just the way things work, with the absence of optical effects a pure mineral is going to be just one color. Quartz is famous for being colorless, for instance, while the vast majority of calcite is white.
Even then, impurities can cause different colorations within the stone. The amount of iron required to turn quartz yellow or purple (ie: citrine or amethyst) doesn’t actually affect the crystalline structure, so you end up with perfectly formed crystals that have different colors.
Each different color in a complex stone is a different compound. How much they differ depends on the stone.
In the case of something like a jasper, the whole stone may indeed be cryptocrystalline, opaque silica but differing levels of impurity can lead the surface of a seemingly homogenous stone to have complex coloration.
It’s important to understand that stones aren’t minerals.
Instead, they’re a conglomerate of a bunch of individual compounds (which we usually call minerals) so complex color patterns are just a matter of impurities being unevenly distributed throughout the material.
Impurities are distinct from separate minerals: they’re microscopic amounts of material that don’t have a large overall effect on the physical properties of the stone, like hardness and density. When trapped, however, they can produce an astounding variety of colors.
Beryl is a great example of this principle. In its purest form, beryl is a colorless mineral with a hexagonal crystal structure. Add a bit of chromium and vanadium and you’ll get an emerald, while manganese produces morganite. Add iron and you get aquamarine or heliodor, depending on the particular Fe ion that ends up in the crystalline structure.
Stones can also change color depending on the heat they reached during formation, or even afterward. Amethyst is a prime example: a small temperature difference is all that separates it from prasiolite or citrine, but they’re all colored by iron.
Some stones also receive radiation from various sources while in situ. Radiation can alter the ions trapped in a stone, creating different colors as well.
So the main factor in stone coloration boils down to the minerals a stone is comprised of and the impurities contained within them. The different forms of atomic material present in the stone can also change the color, such as the difference between Fe₂+ ions and Fe₃+ ions that change beryl from aquamarine to heliodor depending on the form within.
Even in cases like malachite, which is colored by copper, the variation in color along the surface is caused by differing amounts of minerals in different layers.
How Impurities Affect a Stone’s Color
Not all impurities have the same effect on each stone, but you can usually draw some conclusions based on the color. Especially when you’re talking about various parts of the same family, such as chalcedony, agates, and jaspers.
But other times the effect of an impurity can cause a big difference in the end color of the stone.
For instance, chromium impurities are known to create bright green colors in many stones. Emeralds, chrome diopside, chrome chalcedony, and chrome tourmaline all have various dark shades of green.
Chromium in corundum, however, will turn it red and create rubies. These two colors are on the opposite ends of the color wheel, it just depends on how they affect light’s interplay with the crystalline structure of the stone.
Coloration is the reflected light that comes off of an object. Most of us know how prisms work: by differentiating the light into separate wavelengths. Different crystals will have different internal structures, creating different reflection levels.
Whichever light wavelength reflects the best is what you’ll see.
Some common impurities that cause colors in stones are:
Most base elements can cause some coloration if they end up in the structure of a stone.
In stones like agate, water is saturated with silica deposits over prolonged periods. The water may pick up different trace elements as it works its way through the aquifer, which may also be responsible for the banding of the agate.
In many cases, it’s just a straight deposit. You may have noticed a lot of quartz has orange staining on the sides. That’s rust, the same as you’ll get on a nail left exposed to the elements. It’s just deposited ionic iron over a long period.
Most pure minerals don’t have much color. Quartz, in its purest form, is water clear and so are beryl, corundum, and many other macrocrystalline materials.
In the end, most stones will be colored by impurities in the minerals that make them up. These impurities are introduced as the stones form over geological periods of time.
If you’re studying an individual stone, it’s a good idea to learn how different impurities affect the minerals within it.
Inclusions in Clear Stones
While we’ve touched on the actual color of the stones, many stones also have a clearly visible internal structure. Moss and plume agates are the classic examples, but you can find all sorts of strange inclusions in quartz, chalcedony, and other clear stones if you look around.
These inclusions are usually caused by the partial or full formation of crystals inside of the stone. Often these are oxidized metals, which are different minerals. They just happened to grow in a place where the water was also saturated with silica, effectively growing crystals within a crystalline compound.
Another good example is garnet-in-quartz. There are examples out there of entire garnet crystals forming inside of quartz.
As a general rule, inclusions are different mineral growth inside of another mineral. Agates provide the most dramatic examples, but full-formed crystals can be found inside of other crystals on rare occasions.
What About Optical Effects?
In addition to brilliant colors, stones can display an outstanding amount of optical effects. These occur due to the internal structure of the stone in most cases, as light is refracted in strange ways.
The technical term for these optical effects is luster.
Chatoyant stones, like Tiger Eye, display a shimmering effect with a single axis of alignment. These stones have an internal, fibrous structure where the minerals have grown in parallel. It’s a relatively common effect, and even rarely shows up in stones such as tourmaline.
Iridescent stones like opal have an internal structure that causes prisms. Opal, in particular, is actually comprised of microscopic spheres that change light as it passes through them. Different configurations lead to a play of colors that seem to twinkle.
You also have stones with a Schiller effect. The Schiller effect can be seen in stones like labradorite, where it appears to play under the surface.
There are also things like asterism to account for, where a stone displays a star-like pattern of radiated light from certain angles. Asterism is caused by the internal alignment of the crystalline pattern of the stone, or by the introduction of other crystals like rutile.
And finally, we have color-changing stones that shift color with the light source. These depend a lot on the stone, but in the case of the most famous stone of this type (alexandrite), it occurs because of partial replacement of impurities within the material.
The takeaway here is that optical effects are usually caused by the actual structure of the minerals, rather than due to any impurities.
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