Advice For Grading Dark Colored Gemstones

Faceted dark colored gemstones can have intense colors but little brilliance. A simple formula can help a gemologist estimate a fair value for these gems.

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By International Gem Society

Question: I need some advice on how to value dark colored gemstones. I have some chrome tourmalines that are windowed to let the light show through. The color is excellent, but there is little brilliance. I also have a nice ruby that has a lot of black areas on the crown. Where the color shows through, it's nice. Still, with 40% or so of the face-up appearance being black, it's not a brilliant stone. I understand the basics of gem grading but I'm not sure how to deal with these. Any help would be greatly appreciated.

Answer: Grading faceted dark colored gemstones can present some challenges. However, a simple calculation can help you estimate a fair value for these beautiful stones.

ruby- Thailand — Ruby, Thailand (2.75). Photo © Joel E. Arem, PhD, FGA. Used with permission.

How to Grade Dark Colored Gemstones

First, grade each stone based on the visible color as if the whole gem showed that color.

Next, estimate the percentage of the gem that is showing black.

Deduct that percentage from the value of the gem.

This drastically lowers the per-carat value of the gems. However, it's the only way to give a fair value to these stones. This is what I do when I grade dark colored gemstones like the ones you describe for my clients. I've had no negative feedback over this practice.

Hope this helps,

Steve Beitman, GG, Central Coast Gem Lab

chrome tourmaline - dark colored gemstones — Chrome tourmaline, like the crystal on this quartz matrix, gets its intense green color from the presence of chromium. This crystal weighs 66.65 cts. Merelani Hills, Arusha, Tanzania. © Rob Lavinsky, www.iRocks.com. Used with permission.

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