Gemstones and Birthstones: Rubies and What Causes the Color?

Painting the Ruby

If you know your Alice (as in Wonderland), you know the Red Queen’s demand that her minions paint all the white roses red. Same thing happens in the gem world. Many gemstones are “white” or colorless in their pure state. Through the eons the crystals spend growing in the earth, they often pick up trace elements which “paint” them the famous colors they are known for. (Gemstone color can be caused and affected by a great many things, but let’s stick with trace elements here for simplicity.) A perfect example of this is ruby, birthstone of July.

ABOVE RIGHT: Colored gemstones are becoming popular as engagement ring stones. Ruby, the color of passion, is an exceptional choice. Courtesy Tom Linenberger, Goldworks, Fort Collins, Colorado.

For rubies, the trace element that turns plain, colorless corundum — the mineral’s generic name–into glorious red is chromium. Chromium turns other gemstones, such as emerald, green. But why should I confuse you now?

2.03 carat cushion-cut natural ruby. Photo by Mia Dixon, courtesy of Palagems.com.

2.03 carat cushion-cut natural ruby. Photo by Mia Dixon, courtesy of Palagems.com.

The reason is this: White light is made up of all the colors of the rainbow. When it strikes or passes into an object, like a gemstone, light breaks up into all its component colors. The gemstone absorbs, reflects, or transmits those different color wavelengths depending on the particular atomic properties of the gem material and any trace elements. Rubies (specifically the chromium therein) absorb everything but the red wavelengths. As a result, red is the color we see when we look at a ruby.

An interesting aspect of the red in rubies that I would guess has to do with the chromium: If they’re heated with a torch — during repair at the bench, for instance — they’ll go green. The first time I saw this, a customer’s ring was under repair. I think I stopped talking in the middle of a sentence. The jeweler glanced at me and said, “Don’t worry. They’ll turn red again when they cool.” They did.

This color explanation is pretty simplified. If you’d like to learn more, or simply want help sleeping on difficult nights, you can check out your back issues of Lapidary Journal; (March 1992, specifically) for my article “Painting the Ruby Red.” Pleasant dreams!


Sharon Elaine Thompson is a GG and FGA who has been writing on gemstone and jewelry topics for Lapidary Journal Jewelry Artist since 1987. She also writes a line of Birthstone Romances under the name Liz Hartley.


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