The colors of rubies and emeralds are so striking that they define shades of red and green – ruby red and emerald green. But have you ever wondered how they get those colors?
I am an inorganic chemist. Researchers in my field work to understand the chemistry of all the elements that make up the periodic table. Many inorganic chemists focus on the transition metals – the elements in the middle of the periodic table. The transition metals include most of the metals you are familiar with, like iron (Fe) and gold (Au).
One feature of compounds made with transition metals is their intense color. There are many examples in nature, including gemstones and paint pigments. Even the color of blood comes from the protein hemoglobin, which contains iron.
Investigating the colors of compounds containing transition metals leads you into some really amazing science – that’s part of what drew me to study this field.
Rubies and emeralds are great examples of how a small amount of a transition metal – in this case, chromium – can create a beautiful color in what would otherwise be a fairly boring-looking mineral.
Minerals and crystals
Rubies appear red because they absorb blue and green light.
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Both rubies and emeralds are minerals, which is a type of rock with a consistent chemical composition and a highly ordered structure at the atomic level.
When this highly ordered structure extends in all three dimensions, the mineral becomes a crystal.
With a theory developed by physicists in the 1920s called crystal field theory, scientists can explain why rubies and emeralds have the colors they do. Crystal field theory makes predictions about how a transition metal ion’s structure is affected by the other atoms surrounding it.
Rubies are mainly made up of the mineral corundum, which is composed of the elements aluminum and oxygen in a regular, repeating array. Each aluminum ion is surrounded by six oxygen ions.
A crystal of corundum looks like this at the atomic level, with the aluminum ions shown as red balls and the oxygen ions shown as white balls. Each aluminum ion is surrounded by six oxygen ions, and each oxygen by four aluminums.
Eigenes Werk/Wikimedia Commons, CC BY-SA
Emeralds are mainly made up of the mineral beryl, which is made from the elements beryllium, aluminum, silicon and oxygen. Beryl’s crystal structure is more complicated than corundum’s because of the additional elements in the formula, but each aluminum ion is again surrounded by six oxygen ions.
Emeralds appear green because they absorb red and blue light.
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Pure corundum and beryl are colorless. The brilliant colors of rubies and emeralds come from the presence of very small amounts of chromium. The chromium replaces about 1% of the aluminum in the corundum or beryl crystal when a ruby…