Artificial Lighting

GIA’s New Cut Grading System

In 1955, Gemological Institute of America’s (GIA) Gem Trade Lab (GTL) began issuing Lab Grading Reports for diamonds. Concept: In order to assign a value to a diamond, you need to know its quality. GIA hit a home-run by creating a standardized system for grading diamonds (the four C’s; carat weight, clarity, color, and cut). Also, with GIA as an industry watch dog, misgrading or misrepresentation by unscrupulous jewelers could be avoided.

At first, it worked. Jewelers knew that if they had a good diamond, the ideal thing was to send it in for a Lab Grading Report. Then a funny thing happened; others realized there was money to be made in having the power to bless or condemn a diamond’s quality, so they started their own labs (EGL, IGI, HRD, AGS, etc.).

The word certificate (see my article, Certifiable: Lab Grading Reports, Are They Just a Piece of Paper?) started being thrown around and it was implied that a diamond didn’t have any value without its “papers.” What the public wasn’t aware of, was the labs do not discriminate which diamonds they issue reports on. Any diamond sent in, regardless of quality, got “papers” (Lab Grading Report) so the jeweler could reference it during the sale.

While I wrote at great length on the current validity of lab grading reports in Certifiable: Lab Grading Reports, Are They Just a Piece of Paper?, this article focuses on a new aspect of Lab Grading Reports that will be incorporated into the reports in the Fall of 2005 by GIA—a cut grading system.

According to Tom Moses, GTL’s VP, computer-generated models were used to determine the most appropriate set of proportions (for round stones) to increase the amount of sparkle (brilliance + dispersion) and scintillation of a diamond to the viewer’s eyes. After the computer model calculations were done, Human Eye Measurement (HEM) was needed to solidify the predictions. It has been reported by GTL that over 65,000 observations were made to quantify if human preferences matched what the computer light tracing experiments predicted would be the most optimal way to cut a diamond. NOTE: While 65,000 observations sounds like 65,000 people were used in the trial, in fact only 350 people were used. Every time they looked at a diamond, even if it was more than once, it was counted as an observation. (One million hits on a web site doesn’t mean one million unique visitors found the site.) A reported 2000 diamonds were used for the calculations.

Regardless, after GTL’s models suggested that the current cutting standards for “Ideal” or “Class 1” were too strict, the 350 participants couldn’t agree with GTL’s conclusions on which diamonds were more sparkly. Instead of going back to the drawing board, GTL blamed the disagreement on poor lighting. They then cranked up the lighting until the observations matched the predictions.

This leads me to an important point; the models appear to ignore mathematicians Tolkowsky and Ditchburn’s work on proportions and light return in respect to their guidelines for maximum and minimum tolerances. If larger table percentages and larger depth percentages are acceptable, it will allow jewelers to sell what was previously considered a poorly-proportioned diamond as a well-proportioned one, as was first reported in the May 16, 2004 article in National Jeweler by Victoria Gomelsky:

“When the system is introduced, it will profoundly change the way that manufacturers cut diamonds and retailers sell them. The latter are among those who are concerned about the trade’s lack of preparedness for such a development. They fear that consumers accustomed to the Ideal Cut will lose confidence in the industry’s ability to agree on the issue of diamonds’ appearance. But supporters say a third-party evaluation of cut will help people at all points of the supply chain sell diamonds previously considered unsalable.”

In the article “Grading the Make” by Rob Bates, Senior Editor of Jewelers Circular Keystone, he writes that the only way GIA could get their numbers to jive was to choose a “standardized lighting environment!”

Unless you’re Michael Jackson, you don’t live in a standardized lighting environment! We live in cloudy days and fluorescent lit offices; sunny days and candle lit restaurants. Any test to determine a diamond’s beauty must consist of multiple lighting environments. The diamond that averages the best under lighting conditions that range from the best to the worst should be declared the winner. That’s how a decathlete is declared the world’s greatest athlete. Not because he’s best in all events, but because his cumulative score in ten events ranks him the best.

Well, GIA is undeterred. The exact parameters of GIA’s new cut grading system are laid out in a new 26 page article (Fall 2004) by Thomas M. Moses and colleagues, “A Foundation for Grading the Overall Cut Quality of Round Brilliant Cut Diamonds.”

One aspect of the new system changes “class” to “category” and adds a fifth category (category five, AKA bottom of the barrel) by sub-dividing class four into two new categories.

I just can’t sign up for the new far-reaching criteria. The laws of physics haven’t been repealed. Reflection and refraction of light from a diamond does not differ today from over 50 years ago when R.W. Ditchburn, mathematician and author of “Light”, did his initial work on diffraction and resolution with non-coherent illumination.

To imply that a diamond can now be cut with crown angles between 27.0 degrees and 38.0 degrees and pavilion angles from 39.8 degrees and 42.4 degrees and still be a category two is misguided. It’s true that with enough light and enough movement of the diamond, any rock will have some pop, but GIA’s methods–standardized lighting; non-use of metrics for scintillation; equalization of polish and symmetry in classifying with fire and brilliance; over-reliance on human observation (subjective); addition of star facet length and lower girdle facet length measurements and durability criteria—are not credible. While I understand that the jewelry industry will benefit from allowing more diamonds into a new category two, it gives the consumer a false sense of value. Even the new category one extends the acceptable crown angles to 36 degrees and increases the pavilion angle to a maximum 41.8 degrees! Of course the old class one’s and class two’s will still get the highest marks on this new scale, yet it allows inferior grades to tout the same rankings. 

If you accept this new cut grading system, buying a well-proportioned diamond is going to get tougher, if not impossible.

 

by Fred Cuellar, author of the best-selling book “How to Buy a Diamond.” More questions? Ask the Diamond Guy®