All text and images Copyright 2005 American Gem Society.
Reproduced with permission.
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Understanding the New AGS
Cut Grading System
by Peter Yantzer, CG
JCK Show Thursday, June 2, 2005. 10:30 to 11:30 AM in Room 105
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Sit Back, Relax, Enjoy, Absorb
This presentation is on the handout disk.
Also on the disk are the cutting guideline charts for the Standard Round Brilliant and the Square Princess Cut with 2, 3 and 4 rows of pavilion chevron facets.
Additional information:
American Gem Society - www.ags.org
AGS Laboratories - www.agslab.com
AGS Advanced Instruments - www.agsaid.com
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The Concept of an Ideal
In April 2002 at the AGS Conclave in Vancouver B.C. we stated:
“An Ideal Cut Diamond performs better than other similarly cut diamonds over the broadest range of usually encountered lighting and observer conditions. Because the lighting and the observer circumstances so greatly effect the perception of a diamond’s beauty, they must be considered in relation to the diamond’s proportions when assessing cut quality."
In the course of our research, we found round brilliants that met the requirements of the above statement. Tolkowsky’s proportion set was one of them.
We historically have been and continue to be “Idealists”.
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Hardware and Software
AGS developed hardware and software that can be used to evaluate diamond cutting quality.
The hardware can be used to subjectively evaluate cut.
The software objectively measures a three-dimensional scan of a diamond and grades the cutting quality.
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Hardware - The ASET
The “Angular Spectrum Evaluation Tool” is a device that gives the viewer a color-coded map of light usage. This is very important because the quality of light that comes from around and above us is not the same.
There are two configurations of the ASET; desktop and handheld.
Desktop Unit
Handheld Unit
What You See in the ASET
What the Colors Mean
In each of these frames, imagine light coming from the colored areas above and beside the person.
Understanding
Blue is the area that typically produces dark contrast areas in the diamond. It is usually caused by the observer’s head. The amount and distribution are important. For a well made round brilliant this is about 18% blue. Princesses generally run less than this.
Red is the area that misses the observer’s body and head and produces brightness in a diamond because the light sources are direct. Highly desirable!
Green is the area that is usually reflected light and of lower quality. Minimize if possible.
Leakage is shown in black or white depending on the configuration of the ASET. Minimize if possible.
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Which diamond is optically better?
[ Answer = 1, due to more desirable red ]
Which diamond is optically better?
[ Answer = 1, due to better red/blue distribution and less green ]
Which diamond is optically better?
[ Answer = 2, due to more desirable red ]
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The Beauty of the ASET
Even though we haven’t yet modeled the Emerald Cut, you can use the ASET to get an ‘at a glance’ assessment of its light performance. Or, the light performance of any shape for that matter.
You don’t need to take any measurements or know any proportions.
Can you imagine how you could use this instrument to both buy and sell diamonds?
And, the ASET works on most mounted goods.
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Using the ASET to Grade Subjectively
The ASET will soon include charts.
Compare the ASET image you see to the charts.
The closest image will put you ‘in the ballpark’ for light performance.
AGS Members will be able to assign provisional cut grades using this method.
Charts for the Round Brilliant and the Square Princess Cut will be released soon. Other shapes to follow.
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Round Brilliant ASET Chart
Macro Example
Round Brilliant ASET Chart
Micro Example
This also happens to be the old AGS 0 proportion range. It’s obvious why the steep/deep top right corner had to go away in the new system.
Round Brilliant ASET Chart
Micro Example
With 40 degrees of obscuration, the shallow/shallow or lower left corner stones start ‘paddling’ and and they have lower dispersion readings.
Round Brilliant Chart
Micro Example
Cosine squared lighting grayscale images with 30 degrees of obscuration.
Square Princess ASET Chart
Example
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Software; “ AGS Performance Grading
The software measures and evaluates 9 of the 11 components of the new AGS Cut Grading System.
It’s objective because it measures accurately and repeatedly.
It evaluates loose stones only.
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The New AGS System
There are 11 components:
Light Performance
Contrast
Brightness
Dispersion
Leakage
Proportion Factors
Tilt
Durability
Weight Ratio
Girdle Thickness
Culet Size
Finish
Polish
Symmetry
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Contrast
Your head and body, the surrounding environment and different lighting environments can produce contrast. Your head is the usual cause of contrast.
Humans are ‘hard wired’to detect edges. Contrast provides us with these edges. Here’s an example of a shape with virtually no contrast and the same shape with contrast. Humans find the image on the right tobe very appealing compared to the left. Other researchers on diamond cut have pointed out that the presence of contrast enhances our perception of diamond brilliance.
Poor distribution of contrast:
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Brightness
The world is lit from above.
If a diamond gathers light from an area where it is most likely to find quality sources of lighting, it will be bright.
The terms ‘brilliance’ and ‘brightness’ are not interchangeable.
The Illumination of Great Diamond Design
The most desirable angular range is 45 to 75 degrees. Why?
Because it misses the observer’s head and body.
This is where the diamond will most likely find direct sources of illumination.
This is the red area in the ASET.
Brilliance is defined as brightness with positive contrast effects.
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Fire Is Caused by Dispersion
The perception of fire is accentuated by the presence of multiple spot light sources.
The perception of fire is diminished by the presence of broad diffuse light.
Dispersion
Since lighting environments change constantly, we measure dispersion.
We divide the diamond into three zones of equal area and measure the average dispersion for each zone.
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Leakage
Small amounts of leakage is essentially inconsequential. Large amounts of leakage, typically occurring in some fancy shapes and some standard roundbrilliant proportion sets, is detrimental.
Leakage is readily quantifiable and can be factored into a grading system.In these color-coded and ray-traced examples, leakage is quantified and shown in white.
It should be noted that leakage is one of the two vehicles whereby brightness contrast effects are produced. The other is the size and distribution of areas that draw light from low angles; “ the greens in these images. These brightness contrast effects can be positive or negative.
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Scintillation - Continuing Research
We continue to research scintillation. Here’s what we believe at this point in time. The trick is in measuring it or creating a metric for it.Scintillation is a function of the double reflection pattern of a faceted diamond. We call these compound mirrors.
With excellent performance to begin with, you might say:
Big compound mirrors = big fire but small scintillation
Small compound mirrors = small fire but big scintillation.
Dynamic contrast + dynamic fire = scintillation.
The new AGS Grading System handles scintillation passively, and depending on the outcome of our continuing research, probably sufficiently.
Here’s how: If the lower girdle facets get too short, contrast becomes a negative factor. If the lower girdle facets get too long, contrast and dispersion suffer.
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Tilt Examples
It’s easy to make tilt charts for the standard round brilliant. Westarted with the Tolkowsky model and verified what it looked like at 18 degrees of tilt. In order to build a grading system, we needed to ‘relax’that number while still maintaining the same ‘look’. 14 degrees of tilt seems to be realistic.
Girdle reflection shown in green
Tilt Charts
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Durability
Traditionally the industry discounts diamonds with extremely thin and very thin girdles. We also do the same. The other durability factor thatwe do not address is shallow crown angles. Industry wisdom says that a diamond with crown angles under 30 degrees is more likely to break under normal wear and tear. GIA currently issues a statement on its reports if the crown angle is less than 30 degrees.
Diamonds with crown angles less than 30 degrees will receive a deduction.
Here’s an example of a round brilliant with shallow crown angles that gets a cut grade of AGS Very Good 2 in the new system:
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Weight Ratio or ‘Spread’
‘Spread’ is an industry term that refers to a diamond’s face up size compared to its weight. You can also call this ‘weight ratio’or ‘millimeter footprint versus weight’.
The classic example is that a fine make 1.00 carat round brilliant cut diamond should have a ‘spread’of about 6.5 millimeters. Naturally, you would want to purchase the largest millimeter stone that weighs the least amount and still performs. Why pay for unwanted weight?
For a one-carat diamond, the previous AGS Ideal 0 proportions allowed for a millimeter ‘spread’ range of 6.30 to 6.57 mm.
We are using the Tolkowsky model as our historical baseline for the round brilliant. This stone will weigh 1.00 carat with a thin girdle at 6.47 millimeters in diameter. Variations exceeding 5% of this will receive a deduction.
We found many princesses with high performance weighing about 1.38 carats at 6.00 millimeters square. Variations exceeding 5% of this will receive a deduction.
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New Girdle Thickness Chart
[ Note: AGS has released an updated Girdle Thickness Chart, found in this article ]
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Culet Size, Polish, Symmetry
Handled exactly the same as the previous grading system.
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New AGSL Report (Princess)
Expression
Triple Zero Format
Candidates
The reason we use the word ‘candidates’ is because the entire system is dynamic. For example, contrast changes with size.
The boundary edges are ‘fuzzy’ in the sense that a cutter can bring a borderline stone into a higher category by adjusting star length and lower girdle heights. A cutter can also adjust girdle thickness to bring some stones into a higher grade if its grade is being reduced by the weight ratio factor.
You can also lower a candidate through sloppy cutting or indexing the facets, especially the upper girdle facets.
To consistently produce a desired cut grade, cutters will have to cut to ‘fat’ portions of the charts.
The Old and the New
55% Table
The steep pavilion / steep crown and the shallow pavilion / shallow crown corners of our existing two-dimensional system will no longer be AGS Ideal 0’s.
How Square is Square
The AGS recently set a standard for squareness.
The range 1.000 to 1.050 to 1 is considered square.
1.051 to 1 or greater is considered a rectangle.
Chamfering Corners or Points to Improve Durability
Dictionary definition of the word ‘Chamfer’:
Transitive verb; chamfered, chamfering, chamfers. To cut off the edge or corner of.
Noun; a flat surface made by cutting off the edge or corner of a blockof wood or other material.
The AGS also set a standard for chamfering:
"For diamonds that have sharp corners or points, e.g. princess, pear,marquise, heart, etc. chamfering is allowed to the extent that it improvesdurability without obviously changing the shape of the stone."
Note: The standard was purposefully meant to be subjective.
