Color quality classification of ruby and blue sapphire by CIELUV and MUNSELL color systems

Abstract

This research studies the color quality classification of the ruby and blue sapphire by Munsell, CIELUV and CIELAB color systems. The gemologists evaluated the color characteristics of gemstones through the visual sensation by matching the gemstones with the Munsell’s color chips under the controlled illumination. Then the HV/C values were converted to the CIELUV and CIELAB color systems. The studied rubies and blue sapphires sets were classified from dark to light into 6 groups by the leading gems and jewelry trading companies. From the u’ v’ diagrams at different lightness levels were classified the color quality by consideration of the distance between the light source and the gemstone coordinates, that shown the color saturation. The longer distance was the higher color quality. From the experiment, it was found that there were four lightness levels of the ruby, L* 20.54 (“Dark Red” and “Deep Red”), L* 30.77 (“Deep Red”), L* 41.22 (“Deep Red”), L* 51.71 (“Vivid Red” and “Pinkish Red”) and two lightness levels of blue sapphire, L* 20.54 (“Dark Blue”, “Deep Blue”, “Vivid Blue”, “Strong Blue” and “Purplish Blue”), L* 30.77 (“Vivid Blue” and “Strong Blue”). The color difference of the gemstone classification was considered from the L* a* b* diagram by the average values. The ruby had tendency to be more red from the “Dark Red” to “Pinkish Red” classifications. The blue sapphire had tendency to be more blue from the “Dark Blue” to “Strong Blue” classifications. And the green color was found in the “Purplish Blue” and “Greenish Blue” classifications.