*The following article is written in collaboration with the American Gem Society Laboratories. Special thanks to Jason Quick and Brock Wilke for providing helpful insights, suggestions and graphics.
The American Gem Society (AGS) established their own diamond grading laboratory (AGSL) in 1996 with a mission to provide consumers a better understanding of diamond cut quality. Since that time they have been on the forefront of this critical aspect of diamond beauty and value, and have led the way in the modern field of diamond light performance analysis.
Light Performance Cut Grading
It is widely agreed that Cut Quality has a greater impact on diamond beauty than any other single aspect. It is also the factor that is least understood in the market because of its complex nature. A faceted diamond is essentially a miniature 3-D sculpture consisting of a system of tiny mirrors. Each of these mirrors makes a contribution to the overall light performance of the diamond. For optimal performance these mirrors need to be precisely placed and aligned in three dimensions. Light performance is the basis for the diamond cut grading system developed by American Gem Society Laboratories (AGSL).
Early systems for grading diamond cut involved trying to predict cut quality by making broad assumptions based on a few averaged 2-D parameters. The problem with this traditional approach is that geometric flaws and asymmetries in the 3-D sculpture are lost in the averaging process, such that twisted and distorted geometry is indistinguishable from a perfectly cut stone. Advances in technology and computing power have made a much more detailed scientific analysis possible in the modern era.
The release of the AGS Performance Grading System in 2005 marked a departure from traditional methods of cut grading and a leap into the future of diamond light performance analysis. Rather than simply taking basic measurements of a diamond and matching them to pre-defined grades in a collection of grading charts, the AGS system performs direct assessment of a 3-D model of the diamond, taking into account the contribution to light performance of every facet. The system utilizes sophisticated light ray tracing software to simulate the precise behavior of 40,000 virtual light rays. The software creates light maps of various kinds, and grading involves analyzing 11 different aspects of the diamond’s light performance and craftsmanship. It is the only scientifically rigorous and peer reviewed light performance system for assessing diamond cut quality. *Links to the published foundational science available at the end of this article.
AGS Light Performance Grading Hemisphere
The AGS system grades light performance by analyzing Brightness, Contrast, Leakage, and Dispersion (fire). Also part of the analysis are proportion factors of Weight Ratio, Durability, Girdle, Tilt, and Culet. Finally, the finish factors of Polish and Symmetry are assessed by trained graders and factored in to the final cut grade. The system uses a 0 to 10 grading scale where 0 is Ideal. (You might also think of it as “zero deductions”).
The AGS Cut Grade Scale
The grading scales can be found on the bottom of an AGSL report. The Cut scale listed first is broken down as follows:
ASET (pronounced like the word “asset”)
A fundamental aspect of the AGSL approach involves understanding how a diamond is acquiring light from the environment based upon its geometry. The Angular Spectrum Evaluation Tool (ASET)
is the graphical basis for the AGSL system. It is a simple yet insightful tool that helps explain various important aspects of a diamond’s illumination, and which is mathematically incorporated into the grading software.
ASET creates a color coded light map based on the angular position of the source of light being returned to the eye. Light coming from 0 to 45 degrees is colored green, from 45- 75 is colored red, and from 75-90 is colored blue.
ASET Hemisphere Side View
In most lighting environments the brightest light is coming from overhead, either from ceiling mounted indoor lighting or the sun in the sky outdoors. Therefore red can be thought of as light directly from the source, generally the brightest light in the environment. Green is light coming from lower on the horizon and is typically reflecting off walls or other objects in the environment, and is generally less bright. When looking at a diamond at close range the light from the very highest angular range is blocked by the head of the observer. This obscuration effect is colored in blue, and as we will see, has important implications for diamond design. Light that enters a diamond and is not returned to the eye is known as Leakage and is represented by either black or white, depending on whether the diamond is backlit or has a black background.
Consistency and Repeatability of the AGSL Methodology
One of the major strengths of the AGSL approach to cut grading is that the analysis is conducted in a standardized virtual environment and determined entirely by mathematics and the laws of physics. The geometric model of the 3-D scan of the test diamond is analyzed inside a precisely structured observational environment. The results are therefore highly accurate, consistent, and repeatable.
The beauty of this approach is that by quantifying the most important aspects of light performance, the AGS system establishes a solid foundation for continuing development. Grading metrics have so far been developed for Round, Princess, Oval, Emerald, and various proprietary cuts including certain Cushions, and cut grade reports are available for each. For an in-depth look at cut grading of a fancy shape, see our article on Princess Cut Grading at AGSL
A Closer Look at the Eleven Grading Aspects Measured
Light Performance Factors
- Weight ratio
Brightness is the quantity and intensity of light that is being returned to the eye. The ray tracing software returns various values for brightness and can color code the angular ranges from which the diamond draws light. Because high angle light is generally the brightest, ASET signatures that have significant amounts of red with some green (lower angle light) and very little white (leakage) receive the top scores for brightness.
Contrast is differential brightness between adjoining facets which contributes to the perception of brilliance, fire and scintillation. Contrast must be present in the proper amount and distribution in order for the diamond to display optimal illumination effects. The high angular range represented by blue in ASET amounts to designed-in contrast. This “obscuration” essentially uses the head and body shadow of the observer to create structured lighting that enhances the appeal of the diamond. Too much or too little contrast can be detrimental.
As the diamond moves the contrast patterns change. This is known as dynamic contrast and has implications for the diamond’s overall performance. It is therefore important to analyze contrast through a range of degrees of obscuration and tilt angles.
Fire is the display of individual colored sparkles produced as a result of light rays exiting the crown of the diamond as they are dispersed into their spectral components and rainbow hues. With its high refractive index diamond is capable of producing exceptional fire in lighting conditions conducive to seeing this prismatic effect. Cutting factors will have the greatest impact on whether this fire potential will be realized in fire friendly environments.
Leakage refers to areas of the diamond that draw light through the pavilion rather than the crown. This is why leakage areas are frequently referred to as windows, as the eye is able see through the pavilion into the world below. Therefore, in order for leakage areas to appear bright, the diamond's pavilion must be illuminated with sources from below and the pavilion must also be unobstructed by the mounting. Since these conditions are rarely met in ordinary viewing conditions, leakage in significant amounts is detrimental to the overall brightness of the diamond and is therefore not desirable. A small amount of leakage is inevitable and can even contribute to positive contrast and scintillation.
In addition to the light maps and analysis of light return in the face up direction, the AGS system takes into account light performance through a range of tilt angles.
A diamond has certain properties when viewed “face up” with line of sight perpendicular to the table of the diamond. But real world viewing involves a dynamic range of viewing angles. So it is important to understand changes in light performance as the diamond is tilted through a range of viewing angles.
Assessing the size of a diamond in relationship to its carat weight (also referred to as “spread”) is important in making sure that the diamond has no significant amount of hidden weight. Since diamonds are sold by the carat, and prices increase as diamonds get bigger, there is an incentive for cutters to retain as much weight as possible, especially when close to “magic” marks such as 1.00ct. There are a number of cutting techniques used to accomplish this. By calculating what the weight of the diamond should be according to its dimensions, it is possible to know if hidden weight is significant and to apply the proper penalty for it.
Expected weights are determined according to mathematical formulas. A diamond weighing in excess of 5% more than its expected weight will receive a weight ratio deduction. For example, a round diamond that is 6.47mm is expected to weigh 1.00 ct. If it weighs more than 1.05ct, it receives a weight ratio deduction.
Girdles that are extremely thin and/or are combined with shallow crown heights create a knife edge that is significantly more susceptible to chipping than a medium girdle with a higher crown angle. Diamonds with elevated durability risk due to proportioning receive a cut grade deduction.
The culet is small facet that is sometimes put on the point of the pavilion. This has historically been done to make the diamond less susceptible to chipping. A large culet will appear as a “hole” in the bottom of the stone where it acts as a window to leak light. If a diamond has a culet that is more than medium in size it will receive a deduction.
Finish – Polish and Symmetry
Polish and symmetry are visual assessments that are graded by human graders. For a diamond to get a grade of Ideal, every facet must have a mirror finish. A diamond may receive a deduction for Polish if the cutter did not take enough care with one or more facets, of if graining or other characteristics of the diamond made it impossible to achieve a mirror finish.
Symmetry is an assessment of the consistency with which adjacent facets come together at the proper locations. It is often referred to as “meet point” symmetry. Diamonds can have symmetry faults due to an off-center culet, wavy girdle or other types of irregularities in the facet junctions that result in deductions.
Optical Precision and Hearts and Arrows
Hearts and Arrows diamonds
are those cut to a high level of precision in three dimensions. This is often referred to as “optical symmetry”, but that term can be confused with the “meet point” symmetry graded under finish - a measure of how consistently contiguous facets meet. A better term than optical symmetry is “optical precision”. This important factor relates to how well corresponding facets are aligned in three dimensional space. Greater optical precision maximizes the light handling capability of the facet design.
While neither GIA nor AGSL grade optical precision specifically, because the AGS system analyzes a 3D model of the actual diamond, deficits in optical precision that negatively impact light performance will result in deductions. In this way AGSL indirectly grades optical precision.
In addition, AGSL Platinum reports are available with optional ASET light maps. Round diamonds with dual light maps will reveal visually the level of optical precision in the diamond. The dual light maps represent essentially the same views you would see through a hearts and arrows viewer.
Arrows View of Computer Generated ASET
Hearts View of Computer Generated ASET
AGSL Platinum Report with Dual ASET Light Maps
(click image to enlarge)
More detail related to AGSL cut grading will be found in future iterations of this article. Please check back from time to time!
To see other articles in this series: