Why are rubies red and emeralds green? Their colors come from the same metal in their atomic structure

Rubies get their bright color from some fascinating chemistry. Matthew Hill/Bloomberg Creative Photos via Getty Images

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.
benedek/E+ via Getty Images

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.
SunChan/E+ via Getty Images

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 or emerald forms underground at a high temperature and pressure.

But how can one element – chromium – create the red color of a ruby and green color of an emerald?

Color science

Rubies and emeralds have the colors they do because, like many substances, they absorb some colors of light. Most visible light, like sunlight, is composed of all the colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. These colors make up the visible light spectrum, which is easy to remember as ROY G BIV.

Objects absorb some visible light wavelengths and reflect others, which is why we see them as having a color.
Fulvio314/Wikimedia Commons, CC BY-SA

One of the main reasons why objects have a color is because they absorb one or more of these visible colors of light. If a substance absorbs, for instance, red light, it means that the red light gets trapped in the substance and the other colors reflect back to your eyes. The color you see is the sum of the remaining light, which will be in the green-to-blue range. If a substance absorbs blue, it will look red or orange to you.

Unlike the colorless aluminum ion, the chromium ion absorbs blue and green light when surrounded by the oxygen ions. The red light is reflected back, so that’s what you see in rubies.

In an emerald, even though the chromium is surrounded by six oxygen ions, there is a weaker interaction between the chromium and the surrounding oxygen ions. That’s due to the presence of silicon and beryllium in the beryl crystal. They cause the emerald to absorb blue and red light, leaving the green for you to see.

The ability to tune the properties of transition metals like chromium through changing what is surrounding it is a core strategy in my field of inorganic chemistry. Doing so can help scientists understand the basic science of metal-containing compounds and the design of chemical compounds for specific purposes.

You can take delight in the amazing colors of the gemstones, but through chemistry, you can also see how nature creates those colors using an endless variety of complex structures made with the elements in the periodic table.

Daniel Freedman does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.