Faceted Gemstones And Light Return Measurements

by International Gem Society

Faceted gemstones are cut to bring out their brilliance. The faces or facets are cut at angles to reflect light back to the viewer. “Spinel, 1.3 carat, round brilliant cut.” © Dan Stairs Custom Gemstones. Used with permission. — Faceted gemstones are cut to bring out their brilliance. The faces or facets are cut at angles to reflect light back to the viewer. “Spinel, 1.3 carat, round brilliant cut.” © Dan Stair Custom Gemstones. Used with permission.

Question

Has anyone measured the light return from actual faceted gemstones? This seems like it would be a straightforward study, but there would be a subjective element to the study, too. For example, poorly cut stones with very bright outer rings and “fish eyes” look terrible. However, they might return more light in total than faceted gemstones with a more uniform return.

Answer

To my knowledge, no one has done light return studies of real faceted gemstones. It’s possible, but the number of variables would be astronomical. With a ray tracing program like GemRay, you can assume that the facets are flat and the polish immaculate. When working with actual stones, these factors would affect the results of the study. The biggest problem would be color saturation. The fact that a gem has color means that some of the light entering it is absorbed.

To do a study like this, you would need a set of colorless faceted gemstones that were perfectly cut and polished. You would then need sets of colored gemstones, also perfectly cut and polished, with calibrated levels of saturation. Plus, you would need these sets cut to different faceting designs. The results you get from a round brilliant set don’t directly equate to other shapes and cuts. Now, multiply all these measurements by how many angle combinations you want to test. Not to mention how many different gem species. You don’t cut garnet the same way you cut aquamarine. You also don’t cut a 10-carat garnet the same as you would a 1-carat piece.

“Garnet Ring” by Paul the Jewelry Artist is licensed under CC By-ND 2.0. (Cropped to show detail).

Using real faceted gemstones, this project would get really complicated really fast. With a ray tracing program, you can change the angles of the facets and refractive indices with ease.

Ray tracing programs have improved our understanding of light behavior inside faceted gemstones. However, it’s still short of being a science. We still have to choose our cuts and angles based on our experience.

Donald Clark, CSM IMG