The Performance-Based Cut Grading System:

The American Gem Society Laboratories team began researching diamond cut 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. This is the most technologically advanced, yet easy-to-use, 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 observes the girdle in all areas for an extremely thin or very thin area. This is because the non-contact measuring device might inaccurately scan these areas. Finally, the grader observes the culet for damage. If it is chipped, the grader notes this under clarity.

The performance-based American Gem Society Cut Grading methodology will facilitate the development of cut grading systems 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. The square modified brilliant - also known as the princess cut, the square step cut, the rectangular step cut - known as the emerald cut, the octagonal step cut, and the oval round brilliant are currently included in the AGS Diamond Grading Standards for cut.

Additional education is available at various industry events and/or Guild meetings.  American Gem Society experts deliver presentations on the cut grade, the AGS Performance Grading Software, ASET, and the grader’s role in the grading process.

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 866-805-6500 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.

 

Grading Mounted Diamonds for Performance

AGS Diamond Grades (0-10) can be assigned only if a diamond is graded when it is unmounted. This is because a mounting of any type will limit the grader’s view of the diamond, thereby reducing the accuracy of the observations and measurements on which grades are based. Additionally, a mounted diamond cannot be placed on a scale and weighed.

In order to perform an appraisal, an AGS titleholder can grade a mounted diamond according to AGS Standards and assign an AGS Provisional Grade. The diamond’s carat weight also can be estimated by taking measurements and using these in a mathematical formula.

There are three main restrictions on AGS Provisional Grades:

1. They may be applied to mounted diamonds examined for appraisal purposes only.
2. Mounted diamonds cannot be graded a 0 in cut, color, or clarity.
3. The following Statement of Limitations must be included in the appraisal document:

Mountings prohibit the full and accurate observation of cut, color, clarity and weight of gems; therefore, only a provisional grade may be assigned. No diamond may be assigned the AGS Grade of 0 (zero) in cut, color, or clarity unless it is graded unmounted.

These restrictions apply only to diamonds that are graded after they have been mounted. If a diamond is weighed and graded when it is loose, it keeps its AGS Grade after it is mounted (unless it is damaged during setting or wear).

 

Performance Grading

To grade a mounted diamond for performance, according to AGS Standards, the following instruments and equipment are required:

1. Angular Spectrum Evaluation Tool (ASET)
2. ASET Image Charts (*click link below)
3. Microscope — A binocular gem microscope equipped with 10x magnification, darkfield illumination, and overhead fluorescent light

The provisional approach to cut grading is based largely on visual estimation and assessment techniques that have been part of diamond cut grading for many years. An important innovation is the use of the Angular Spectrum Evaluation Tool (ASET^®) and ASET^® Image Charts to assess performance.

With the provisional approach, ASET Image, Proportions, Symmetry, and Polish are evaluated separately, and the lowest individual comment sets the Provisional Performance Grade.

The following 4-step procedure applies to standard round brilliants only:

1. Evaluate the ASET Image
2. Estimate Table%, Crown Angle, Girdle thickness, & Culet size
3. Evaluate Symmetry and Polish
4. Assign the AGS Provisional Performance Grade

Evaluating the ASET Image

The ASET image provides a quick and easy way to assess a diamond’s interaction with light. It’s brightness, leakage, and contrast can be evaluated by noting the pattern and amount of red, blue, green, and black.Thus, the ASET image provides a reasonably effective way to analyze Light Performance when more precise methods cannot be used.

To evaluate the ASET Image:

1. Look at the diamond carefully, and compare its appearance to the illustrations in the ASET Image Charts.
2. Using the chart illustrations as a guide, select the appropriate comment to describe the overall level of light performance shown in the ASET image:

High (H)

Medium High (MH)

Medium (M)

Medium Low (ML)

Low (L)

For complete information on the AGS Diamond Grading Standards and the entire procedure for provisional grading of mounted diamonds (cut, color, clarity, and carat weight), see The AGS Way course and Diamond Grading Manual. This is available through the Education department at 866.805.6500 extension 1034. There is special pricing for existing titleholders.

For more information on light performance please visit the Advanced Instruments Division website.