Professional Jeweler Archive: The Human Factor in Diamond Cut: GIA's Ongoing Study

October 2003


The Human Factor in Diamond Cut: GIA's Ongoing Study

The Gemological Institute of America is finally putting to rest the howls of critics who said it relied too much on computers and not enough on real-world human observation in its ongoing study of the impact of cut on diamond appearance. In an interim report in the Rapaport Report, GIA’s research department says it has gathered 45,000 human observations using 300 individuals (who mostly work in the trade) on more than 1,300 real diamonds.

These human observations confirm GIA’s earlier findings that many different cut combinations yield high brilliance and fire. The human tests show a fairly wide range of crown angles and/or table sizes can lead to beautiful diamonds when balanced by other suitable proportions. They also strengthen GIA’s belief that every facet matters when it comes to judging diamond appearance, even “minor” facets, such as star or lower girdle facets.

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

GIA researchers say human observation 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, the brilliance metric now takes into account observations by people in the trade using actual lighting and viewing environments and angles. In the case of fire, GIA incorporated 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 also helped GIA understand just how much the appearance of a diamond is affected by changes in lighting and viewing environments. Among factors 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 write.

As a result, GIA developed a controlled viewing environment to evaluate diamond cut that incorporates real-world lighting under three conditions:

  • Fully diffused light to observe brilliance.
  • Spot lighting that emphasizes fire.
  • Mixed lighting to evaluate 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 cross-section of manufacturers, dealers, retailers and the public can use to evaluate diamond cut.

GIA is to be applauded for its impeccably impartial scientific approach to this long and arduous study, which includes listening to a lot of feedback about its work. The fact GIA continues to take the time to study every aspect of this complicated issue before issuing a definitive statement on cut, despite the clamor of so many for answers, says a great deal about its professionalism. The institute understands clearly it’s more important to be right than to be fastest or first.

– Peggy Jo Donahue

Copyright © 2003 by Bond Communications