The Performance-Based Cut Grading System:
The Performance-Based Cut Grading System:
The American Gem Society Laboratories team began researching diamond cut in 2000, working closely with Dr. José Sasián, an optical physicist, 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. 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 determining the Light Performance. Durability, Tilt, Weight Ratio, Girdle Thickness, and Culet Size are considered in evaluating the 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.
Using magnification, the diamond grader evaluates the Symmetry and Polish details of the diamond and assigns a grade. 0 (best) – 10. The grader also verifies the condition of the girdle and culet.
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American Gem Society Diamond Chart |
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.
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, you can use it to gain a general idea about a diamond’s performance. For more information about this innovative tool, please contact an American Gem Society Jeweler.
Definitions:
Brightness: The amount of white light returned to the observer. (Brilliance is Brightness with positive contrast effects.)
Dispersion: The separation of white light into spectral colors.
Leakage: Areas that do not return light.
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.
Durability: The diamond’s resistance to chipping or breaking.
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 diamond's face up size versus its weight.
Girdle Thickness: The width of the girdle measured all the way around the diamond.
Culet Size: The size of the facet at the termination point of the diamond. A sharp point or no culet is acceptable.
Polish: This is the condition of the diamond's surface and facet edges.
Symmetry: This is the shape, placement, and alignment of the facets
