The Performance-Based Cut Grading System:
The Performance-Based Cut Grading System:
The American Gem Society Laboratories team began researching diamond over five years ago, in 2000, working closely with Dr. José Sasián at the University of Arizona in Tucson and other experts in diamonds, optics, and computer programming. The result is a new methodology that analyzes the light performance and proportions of a three-dimensional image of a faceted diamond. The previous proportion-based grading system for the standard round brilliant cut only addressed averages for the table diameter percentage, the eight crown main facet angles, and the eight pavilion main facet angles. Therefore, it was two-dimensional in nature. With the new performance-based cut grading system, the optical effects of all the facets are evaluated. It is no longer sufficient to evaluate a diamond's performance just by measuring and maintaining certain proportion sets - such as table %, crown angle, or pavillion depth. The proportions of a diamond will drive its performance, meaning proportions are important - but various percentages and angles have been found to produce a beautiful display of fire and brightness. This is the most technologically advanced, yet easy-to-understand, cut grading analysis available today.
There are three categories in the new AGS Cut Grading System — Light Performance, Proportions, and Finish. Within these categories are eleven factors:
*Brightness, * Dispersion, * Leakage, * Contrast, * Durability, * Tilt, * Weight Ratio, *Girdle Thickness, * Culet Size, * Polish, & * Symmetry
Brightness, Dispersion, Leakage, and Contrast are considered in Light Performance. Durability, Tilt, Weight Ratio, Girdle Thickness, and Culet Size are considered in Proportions. Polish and Symmetry are considered in Finish.
The AGS Performance Grading Software objectively measures and evaluates nine of the eleven grading factors. The remaining two — Polish and Symmetry — must be evaluated by a diamond grader. At the software’s core is a robust optical ray-tracing engine, which accurately measures Brightness, Dispersion, Leakage, and Contrast. In addition, the software calculates the volume of the object, determines its Durability, Tilt, and Weight Ratio. The Girdle Thickness is measured in sixteen areas known as the peaks and valleys, and the Culet Size is measured.
The software evaluates the eleven factors, separating them into two different types of deductions — cumulative and net lowering — for each factor. The cumulative deductions are totaled from Brightness, Dispersion, Leakage, Contrast, Durability, Tilt, and Weight Ratio. The net lowering deductions are individually considered for Girdle Thickness, Culet Size, Polish, and Symmetry. The software adds the cumulative numbers and then compares the sum to the individual net lowering deductions. The largest number sets the cut grade.
The Role of the Diamond Grader
Using magnification, the diamond grader evaluates the Symmetry and Polish details of the diamond and assigns a grade from 0 –10 for each. The grader also verfies the condition of the girdle and cutlet.
The AGS ASET
The AGS ASET provides a quick and easy — yet sophisticated — way to assess a diamond’s interaction with light. The name ASET stands for Angular Spectrum Evaluation Tool, and the device was developed to be used with the performance-based American Gem Society Diamond Cut Grading System. The tool’s design and the procedures for using it are simple. However, the results reflect one of the most important discoveries to come from the research for the current performance-based system. Handheld and desktop models are available. The ASET does not supply all the detailed information that can be obtained from an optical measuring device combined with the AGS Performance Grading Software. However, with a little practice, a jeweler or buyer can use it to gain a general idea about a diamond’s performance. The ASET also is a great sales aid. In a retail presentation, it instantly can clarify some of the most critical technical points concerning cut quality. For more information about this innovative tool, please contact the AGS Advanced Instruments Division, at 702.255.6500 extension 1030.
Summary
The new performance-based American Gem Society AGS Cut Grading System will facilitate the development of cut grading methodologies for any shape and facet arrangement. The industry will be able to optimize existing and patented cuts, as well as design and optimize new cuts. Currently, the standard round brilliant, the square modified brilliant – described as princess cut, the square emerald cut, the rectangular emerald cut, the octagon step cut, and the oval brilliant cut are being evaluated. Research continues on additional styles of cut.
If you would like to use the AGS Performance Grading Software, you must have access to a non-contact measuring device, a computer, and an Internet connection. Contact the AGS Advanced Instruments Division at 702.255.6500 extension 1030 for further details.
Definitions:
Brightness: The amount of white light returned to the observer.
Brilliance: Brightness with positive contrast effects.
Contrast: The light and dark patterns seen when observing a diamond. It can produce a positive or negative optical effect. This usually is caused by, but not limited to, the observer’s head.
Dispersion: The separation of white light into spectral colors.
Durability: The diamond’s resistance to chipping or breaking.
Leakage: Areas that do not return light.
Scintillation: The combination of white and colored sparkles seen as the diamond and/or the observer and/or the light source(s) move.
Tilt: The point at which the girdle reflects under the table of the diamond and is defined by the minimum pavilion angle allowed for each table size.
Weight Ratio: The millimeter footprint of a diamond versus its weight.
