For each stone, we have measured the transmittance spectrum. The selected stones were a synthetic ruby (corundum), a synthetic aquamarine (beryl), a topaz, an amethyst (quartz), and a citrine (quartz). In this article, we present an experimental setup, which aims at evaluating the overall appearance of a set of gemstones, considering some of their visual characteristics like, for example, color coverage, brilliance, and dispersion. In fact, gemstones cuts are on purpose planned to refract light in many different directions, in order to enhance spatial differences, while minimizing light transmission, and obtain the desired shining effect. The main reason to explain the lack of standards for the evaluation of color in gemstones is related to their physical/optical characteristics. The necessary personal expertise is given by years of practice in the field, because a common observation protocol does not exist. Gemstones color assessment is nowadays mainly based on the direct observation performed by experts by eye or using lenses and microscopes. Results report the correlations between instrumental measures and the visual assessment of gemstones color appearance and can be a potential base for a future standard method for gemstones color assessment. We set up a perceptual experiment involving 21 participants to correlate the measured and the perceived colors of a set of gemological samples.
In this study, we investigate the relationship between the use of color measuring instruments and the visual inspection method for the determination of gems color appearance. What needs to be assessed is the overall appearance of the gemstone and this explain why, so far, it is realized mainly by visual inspection. This puts the process of gemstone color assessment out of the scope of classic colorimetry.
The underlying problem is that light impinging gemstones, due to their physical characteristics, produce scattering and multiple reflecting phenomena. Nowadays, there is a lack of standards for the determination of color of gemstones, and traditional color assessment is exclusively based on the direct visual observation in some cases with lenses and microscopes. A precise and shared description of gemstones color could greatly help their classification and market evaluation, and the key to develop an efficient color evaluation system for gemstones is the creation of a common descriptive language and assessment method.