Can a computerized, multi-channel spectrometer transform the intricate art of measuring the color of gemstones into a more precise, efficient science? Gubelin Gemological Laboratory thinks it has the answer.

            One of the busiest booths at the recent European Watch, Clock and Jewelry Fair in Basel belonged to Gubelin Gemological Laboratory. The prestigious Swiss lab generated the interest by unveiling a new color measuring system that it claims will revolutionize the color grading of gemstones—particularly of diamonds.

            The system is based on the spectrometer, which of itself is not a new development. The German optical firm of Zeiss introduced the spectrometer in 1984 for color measurement in the automobile industry, and the gemstone trade soon adopted the machine.

            While it provides a backup for the human eye’s analysis of color in gemstones, the spectrometer is not infallible. “The technology is problematic,” said Dr. Adolf Peretti, the Gubelin scientist demonstrating the new system at the Basel fair. “It takes hours to get a color determination and there is a high margin for error. Our new multi-channel spectrometer takes only minutes, and it is much more precise and accurate.”

            Gubelin spent two years developing the new system in conjunction with Zeiss. “It’s very upgraded compared to an ordinary spectrometer,” Dr. Peretti explained. “Before, each color was separately analyzed. It took about one minute per color. The multi-channel spectrometer uses a completely new technology to simultaneously analyze the entire spectrum of color within one millisecond.”

            The machine is simple to operate as demonstrated by Dr. Peretti. The gemstone is placed on a sample holder and then subjected to a flash. Fiber optics transmits the light data to the multi-channel spectrometer where it is analyzed. The multi-channel spectrometer is connected to a computer which almost instantaneously presents an analysis of the data on a screen.

            ProfW Boguth, a physicist specializing in optic interpretation, developed the necessary computer software for the system.

            The result is a precise number assigned to the color of a gemstone, as opposed to a name. “What can you do with number? Actually, not much,” Dr. Peretti admitted. “What you need is a gemologist behind the numbers to interpret them. But this system helps a gemologist by always providing the same result. It allows for precise reproduction of data.”

            Other tests, such as subjective color grading by visual appearance in a defined light source and masterstone comparison, remain important to back up the mechanical system. Dr. Peretti stressed.

            “Objective color measurement provides us with numbers that have to be compared with the visual results. Combining conventional methods with new ones will increase the reproducibility and accuracy of color grading of gemstones. A machine should never replace visual grading or masterstone grading,” he added. “But sometimes master stones will not be available and the subjective color grading will depend on the grader as well as the light. I hope the machine will help in such situations as an additional test.”

            The system could revolutionize the diamond grading industry, Dr. Peretti said, since very slight differences in color can dramatically affect the price of a stone. “Now the color grading of diamonds is subjective,” he said. “It depends on the light you use. It depends on the person who’s doing it. How can a consumer who buys today be sure that the diamond will be graded the same tomorrow?”

            The new system can also benefit the trade, Dr. Peretti added. “If somebody in New York has a diamond that he wants to match exactly for earrings, he can measure the color of this diamond and get an exact number which he can transmit to his friend in Tel Aviv.”

            Dr. Peretti said Gubelin has already tested the multi-channel spectrometer with fancy diamond dealers. “They are convinced that it helps them. They want to buy it.” The system is so new that Gubelin has yet to determine a price for it, he added.

            “For us, this system works and if others want to use it, we will help them to. If some don’t accept it, it’s their choice. We won’t point the finger at them.”

            The multi-channel spectrometer will make its US premiere at the GIA International Symposium in June. Dr. Peretti does not expect a warm reception.

            “We are waiting for a lot of criticism,” he said. “Every new development is bound to be difficult. But in the end, the aim is to protect the consumers and not serve the laboratories. Technology must serve the market.”

            While the system is effective with diamonds, it has its limits with colored gemstones, Dr. Peretti added. “For high-luminescent rubies it is not very well applied. There is a potential application, but it has to be tested with a large number of stones and very scientifically.”

            Yellow sapphires also require more research, although emeralds, spinals, garnet and blue sapphires show good results, Dr. Peretti said.

            Dr. H A Hanni, director of the Swiss Gemological Institute, commented that he doubted whether many laboratories will be able to afford to invest in the multi-channel spectrometer, at least initially.

            “I think people will wait to see how powerful it is. Time will show if it’s what we need. Personally, I would like to try this machine, to see if it’s really reproducible, if the same color result always comes out.”

Getting technical

            Previous attempts to develop mechanical systems for the color measurement of gemstones have proven inefficient and unreliable. JewelSiam asked Dr. Adolf Peretti some specific questions in regard to past problems with mechanical analysis of gemstones, and how the new Gubelin multi-channel spectrometer deals with them.

JewelSiam: Does the system utilize an integrating sphere to collect light? If so, would it be correct to say that the instrument measures body color, rather than giving a measurement that can be correlated to the actual appearance of a faceted stone?

Adolf Peretti: The collection system for the transmitted light is a fiber optic system (opening below the table of the stone). The integration sphere has been introduced for two purposes:

            1) to produce a diffuse illumination (eg, the light enters from different directions through the pavilion):

            2) to collect reflected light (either from the surface of the stone or internal reflection) and to send it again as diffuse light through the stone.

            Thus the instrument measures body color as well as the color resulting from multiple reflections. In other words, the color in the tie of a navette diamond is different from those measured through the tip.

            Interpretation of the data is based on known samples. Thus, computer data are correlated to what the eye is seeing. It was found that color data of gemstones of a certain shape and cut (such as navette, mod brilliant) can only be compared to gemstones of the same cut and shape.

JS: In regard to diamonds, how is the problem of brown (i.e., non-Cape series) stones dealt with? How about fluorescent stones? Can the instrumental measurements be related back to the visual appearance of the stone?

AP: Brown and gray series are interpreted as following: The saturation of a color (hue) is measured as well as the tone (amount of gray). Brown colors are due to increase in gray in the color families yellow, orange and pink.

            The ratio of saturation/gray in a stone is determined. Again, the comparison with known samples shows us how high the ratio (saturation/gray has to become before the diamond has a brown modifier or even has to be called brow.

            Fluorescence is measured as well by the machine because the simultaneous multi-channel detection allows us to see the luminescence of higher energy light to lower energy wave length.

            The effect of UV fluorescence to grading of white diamonds was seen by the machine as in the subjective experiment. Diamonds appeared to be F-grade in artificial light and to be E-grade in northern daylight due to luminescence effect. However, more discussion about luminescence needs concrete examples rather than a simple answer. I hope to deal with more problematic stones to discuss this point further. Research with strongly luminescent rubies is not yet complete.

JS: In regard to both diamonds and color stones, how is the problem of color zoning and pleochroism dealt with? I am talking about relating phenomena such as pleochroism to actual appearance, not just inserting a polarizing filter to analyze transmitted light.

AP: The illumination is diffuse light (mainly from pavilion) and viewed through the table (3mm opening).

            During measurement, the stone can be red positioned and after a multiple experiment, the computer calculates the average color resulting from information through all directions in the stone (e.g. from different positions in a navette) if required.

            Diffuse light collects color of different directions in the stone. Thus, pleochroitic colors are superimposed to give a general color (introduction of Pol-filters possible but not your questions). Collection of light through the table and diffuse illumination through different directions in the pavilion resulted in calculated colors that are similar to those observed by eye (sapphires, emeralds, alexandrite, and Paraiba tourmalines). How-ever, the application of measurement of non-pleochroitic icotropic gemstones (such as diamonds, garnets and spinals) is much simpler and still complex enough.