GIA Says Human Observation Confirms There's More Than One Way to Cut a Beautiful Diamond

September 10, 2003

GIA Says Human Observation Confirms There's More Than One Way to Cut a Beautiful Diamond

The Gemological Institute of America's Research Dept. says it has now conducted extensive human observation tests to support and validate the results of its computer modeling project to determine the impact of cut on diamond appearance. The 45,000 human observations GIA has gathered, using 300 individuals (who mostly work in the trade) and more than 1,300 diamonds, continue to confirm its earlier findings that there are many different cut combinations that yield high brilliance and fire.

The human observation tests also strengthened GIA's belief that every facet matters when it comes to judging diamond appearance. They also confirmed the importance of "minor" facets, such as star or lower girdle facets. Finally, observation testing showed that a fairly wide range of crown angles and/or table sizes can lead to beautiful diamonds when balanced by other suitable proportions.

GIA has more work to do concerning scintillation effects, but it says it still plans to eventually incorporate its diamond cut appearance research into expanded information on GIA Diamond Grading Reports for round brilliants.

In the interim report on their human observation tests, which was published in the Rapaport Diamond Report, the researchers say the tests have allowed them to "bridge the gap between theoretical computer modeling and trade experience, each of which has its own strengths and weaknesses." As a result of the tests, GIA has updated its computer metrics. For example, its brilliance metric now takes into account brilliance observations made by people in the trade, using actual lighting and viewing environments and angles. In the case of fire, GIA has been able to incorporate into its metric the point at which the average person's vision is capable of seeing differences in the appearance of colored light.

The human observations have also helped GIA understand just how much the appearance of a diamond is affected by changes in lighting and viewing environments. Among the things that make a difference: different types of lighting, the distance and angle of the light and the observer from the diamond, the color of the observer's clothing, the color of the diamond's tray or mounting, and the color of surrounding walls and objects. "This is why a diamond that looks fiery in retail store lighting may look bright but not as fiery in office lighting," the researchers wrote.

As a result, GIA has now developed a controlled viewing environment for evaluating diamond cut that incorporates "real world" lighting under three separate lighting conditions:

  • fully diffused light for observing brilliance
  • spot lighting that emphasizes fire
  • mixed lighting for evaluating overall appearance.

GIA plans to offer a controlled viewing environment to the trade that will define and support standardization in this crucial area. It hopes this will lead to a useful lighting standard that a broad cross section of manufacturers, dealers, retailers and the public can use to evaluate diamond cut.

For more information, background and details about GIA's ongoing research into diamond cut, go to the GIA Research section at

by Peggy Jo Donahue

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