Gemstone color owes a little to pigments and a lot to light. Some gems get their color the same way paints receive their color. For example, if you wanted some blue paint for your bedroom, the store clerk would take white paint and add a few drops of blue coloring to it. We call this kind of gem “idiochromatic.” The gemstone color is actually in the stone. For example, if you were to grind up a piece of malachite, the powder would be green.
Most gemstone colors don’t work like paint. Instead, most gems gain color from the way light reacts to their crystal structure. If you were to grind up a sapphire, emerald, garnet, or tourmaline, you would get a white powder. It does not matter what color you see when they are whole, or how dark the color is. The color actually has nothing to do with the physical matter, as a pigment in paint. Instead, it has to do with how light reacts to the crystal.
As you know, white light has all the colors of a rainbow. You can see this when a sunbeam passes through a prism. What comes out is a rainbow; which is white light spread out into its component colors.
When light enters a gem, some of it is absorbed. What comes out is no longer white. That is because light has to contain the full spectrum of the rainbow to be white. If some is absorbed in the gem, what is left has to be the gemstone color.
We call this process “selective absorption.” Each gem absorbs a different part of the light spectrum, a different part of the rainbow, which is why gems appear to be different colors.
This is not a simple concept. Garnets are not red because they absorb everything except red light. If that were the case the gems would be very dark, as red is just a small part of the whole range of color. What happens is that our eyes take this mixture of light and blend it to a single color. Rubies are red and emeralds are green to our eyes, but not if you were to break down the color into a full spectrum.
To see what colors of light a gem absorbs, we use an instrument called a spectroscope. Here are two pictures of how light looks in a spectroscope. The first one is white light. You can see the whole spectrum of the rainbow when looking at white light in a spectroscope.
The next picture is the light coming out of a garnet. The dark bands are where light is absorbed. Do you see how little that is? That is why we said that, if only red light came out of the gem, it would be very dark. Actually, most of the light comes through a garnet, so they make brilliant gemstones!
- Allochromatic: Minerals that receive their coloring by selective absorption, by how light reacts to the crystal structure.
- Idiochromatic: Minerals that have their color in the stone itself.
- Selective Absorption: The process where some light is absorbed by a crystal. The light that leaves the crystal is then something other than white; a color.
- Spectroscope: An instrument that separates white light into its range of color.
Gemologist Test Questions
- White light contains the full spectrum of the rainbow. True or False?
- What do you call the process where some light is absorbed when it enters a crystal or gem?
- Emeralds are green because they absorb everything except green light. True or False?
Gemologist Test Answers
- True. If it does not have the full spectrum, it would not be white.
- Selective Absorption.
- False. Green is just a small part of light. If only green came out of a emerald, it would be very dark!