The 4 C's - Diamond Cut Analysis
By Bryan Boyne (g.g.) , Thursday, September 08, 2016
There are two approaches used to analyze diamond cut quality. Proportion assessment utilizes a 2 dimensional approach that takes the outer measurements of a diamond and predicts how it will handle light. Performance assessment measures the actual light output of the diamond and compares it to established standards.
The Gemological Institute of America (GIA) grades the cut of round diamonds only. The GIA system consists of 5 grades: Excellent, Very Good, Good, Fair and Poor. The system takes into account seven factors: brightness, fire and scintillation (appearance based aspects) and weight ratio, durability, polish and symmetry (craftsmanship aspects).
The American Gem Society (AGS) grades the cut of rounds and some fancy shapes based on light performance with scores from 0 (Ideal) to 10. This system takes into account eleven factors: brightness, dispersion, leakage and contrast ( Light performance aspects) and girdle thickness, culet, weight ratio, durability, and tilt (proportion factors) and polish and symmetry (finish aspects).
Since proportions assessment looks at the container and performance assessment looks at the contents we use both methods to analyze our diamonds. Because the AGS cut grading system is more sophisticated and less forgiving, consumers looking for the finest cut craftsmanship prefer AGS reports when available.
A diamond has facets that allow light to enter, become refracted and exit in a rainbow of colors. Certain angles and measurements are proven to maximize these effects. The facet arrangement of the standard round brilliant diamond permits reliable analysis if you know that diamond's proportions.
Note that fancy shapes are more complex and cannot be as accurately predicted using proportions. This is discussed in part 2. Performance Assessment.
Parts of a Diamond
Before proceeding it is important to know the locations of various facets on a round brilliant diamond and become familiar with the role proportions play in light performance:
Round Brilliant Diamond Facet Map
The Tolkowsky Ideal
In 1919 a mathematician and diamond cutter named Marcel Tolkowsky calculated a set of proportions for the ‘Ideal’ round brilliant.
A round diamond cut near these proportions will do a better job of dazzling us with its beauty than one that is not. Those measurements remain a standard by which the world’s finest diamonds are judged and logically formed the basis for the first cut grading system in 1996. As laboratories develop new systems we know that other beautiful configurations are possible, but Mr. Tolkowsky’s work has been largely validated by modern science.
Basic proportions assessment is done by comparing a diamond’s measurements to proven ranges on charts. Over time charts have been developed to predict the potential of a diamond based on its proportions. In 1996 the American Gem Society Laboratory, or AGSL, became the first lab to perform cut quality analysis on the round brilliant diamond, with grading based on proportions near Tolkowsky’s. Since that time the AGS has evolved to a performance assessment system with cut grading extended to certain fancy shapes as well. The AGS ‘Traditional Ideal’ proportions chart is still used as a reference for round brilliants.
AGS Traditional Ideal Proportions, 1996-2005
Round diamonds with overall measurements in this range are considered to have traditional ‘ideal’ proportions. In 2005 the AGS changed to performance assessment, and diamonds at the outer limits of this traditional range may no longer receive their top grade (on the New DQD), but those near the middle will always be considered top performers.
Whiteflash A Cut Above Hearts & Arrows Proportions, 2000-Current
Stricter than traditional AGS proportions, these are the Whiteflash proportion guidelines for A CUT ABOVE® Hearts & Arrows diamonds. Round brilliant diamonds within this range of measurements receive the highest ratings in both proportions and performance grading systems.
Note: A CUT ABOVE® diamonds must first receive 0 ‘Ideal’ grades in light performance, polish & symmetry from the American Gem Society. In addition to passing several additional requirements, imaging tests and visual inspection by the Whiteflash review team, they must also meet the Whiteflash standards for 'True' Hearts & Arrows optical symmetry: How Hearts & Arrows Diamonds are formed
Another type of proportions assessment involves plugging measurements into a cut estimator for a computer assisted prediction of performance.
The Holloway Cut Adviser (HCA)
In the 1990s an Australian geologist named Garry Holloway introduced an online cut analyzer that gained significant popularity. Analyzing 5 basic measurements, it is possible to get an indication of predicted performance.
Click here to evaluate your diamond's proportions with the HCA
Note: As a two dimensional tool the HCA is of limited value when 3 dimensional analyses such as an AGS Platinum report, ASET and other light performance imagery is available.
As of 2006 the Gemological Institute of America provides a similar online cut estimator which ties in to their cut grading system.
Click here to evaluate your diamond's proportions with GIA Facetware
A sophisticated tool used by some diamond cutters and analysts takes a 3D scan of a diamond and generates a virtual model that can be manipulated with specialized software. One such program is DiamCalc by OctoNus. With the help of virtual performance simulations this software enables broad design conclusions to be drawn about various proportion sets.
When evaluating a specific diamond, actual reflector images and comprehensive analysis of the live diamond are preferable to assessment by simulation. However, a great deal can be learned using this advanced tool.
Insist On Knowing
Whether or not you target the most elite diamond performance, it’s reasonable to insist on knowing all critical measurements of any diamond you consider from any seller.
Whereas proportions assessment is a prediction based on measurements, performance assessment looks at the actual light output of a diamond. Diamond performance has traditionally been described in terms of brilliance, fire and scintillation.
Brilliance or Brightness refers to the total amount of light returning from the diamond. Fire or dispersion, is the degree to which light is broken into spectral colors. Scintillation is sparkle - a dynamic effect associated with movement. In recent years other descriptors have been added to enhance our understanding of diamond’s visual properties: Contrast and Pattern describe the arrangement of light and dark areas resulting from reflections. Leakage describes areas that do not reflect light back to the viewer. Methods of performance assessment measure the light output of a diamond and compare it to established standards.
Well Suited To Any Shape
The facet arrangement of round diamonds permits meaningful analysis with simple proportions but fancy shapes are more complex and cannot be decisively predicted in this manner. Performance evaluation is meaningful for every shape, as it reveals light output regardless of outer dimensions. While measurements are still important to know, performance assessment is critical, especially for shapes other than round.
In the 1970s a Japanese scientist named Okuda developed methods to study light performance using colored reflectors in magnified scopes. By color-coding light entering the crown of the diamond he was able to track the amount of light returned properly to the viewer's eye. Over the next 30 years other innovators developed such tools as the FireScope(TM) and the Gilbertson-Scope which were used to improve diamond cutting techniques.
Natural reflector assessment was popularized at the turn of the century with Garry Holloway’s “ideal-scope,” a simple magnifying tube containing a red reflector. An ideal-scope photograph shows which areas of a diamond return light to the viewer’s eye and which areas allow light to leak out. An Ideal Scope image is a handy visual tool for judging diamond light performance.
Angular Spectrum Evaluation Tool (ASET)
In 2005 the AGS introduced a similar tool, the Angular Spectrum Evaluation Tool or ASET. Based on a similar principal, the ASET shows light return with greater detail as light is color-coded 3 ways. Blue (obscuration contrast), red (high angle, high intensity light return), green (lower angle, lower intensity light return)
Mechanical devices have been developed to measure a diamond’s light output. Imagem, Brilliancescope and Isee2 are among the most well-known. These machines provide interesting information that may be useful in the big picture, but each gives different results on a diamond, depending on which company designed it. BrillianceScope and Imagem were created to help stores sell diamonds, not for research or science. For this reason they have met resistance from major laboratories and the science community.
We have tested mechanized/computerized light performance devices but have found none that give relevant and repeatable results. We realize the value of the concept to consumers and may adopt such a machine when it’s made repeatable and the metric correlates to real-world lighting. Until that time we endorse the same natural assessment techniques used by the world’s leading diamond cutters and laboratories.
There are too many different illumination scenarios to try and capture all diamond beauty in simple or mechanical terms, but there is one performance assessment that has stood the test of time: Analysis by a human being. In all cases the “eyes” have it. Numbers and pictures always give way to the eyes of a trusted expert and, more importantly, to the eyes of the one who will wear the diamond.
If you’re shopping on the internet, finding diamonds with traditional ideal proportions is a fast way to locate good candidates. Then you can use reflector images and other performance data to see which has the best light return. Human observation should serve as final confirmation that you have made a great choice.
Some sellers rely on proportions alone, whereas others will post lab reports, onsite measurements, magnified photos, and images of every diamond in a proven natural reflector to show its light output.
Ideal-scope and ASET images are reliable, lab-endorsed methods of interpreting performance potential in a diamond. We strongly believe in this form of assessment as it is a direct measure of the actual light performance.
A positive by-product of Internet diamond sales is that consumers are becoming more educated about what makes a high quality diamond “tick.” Remember that if you are looking in retail stores you should seek the same assessment data and documentation that you would expect on the internet.
GRADING LABORATORIES WE TRUST
The Gemological Institute of America (GIA) bases cut grading on proportions.
GIA uses a scale of 5 levels where the AGS system uses 11. Predictably, the top GIA cut grade, “Excellent,” overlaps several AGS grades. We caution clients that this wider range allows for configurations with proportions farther away from Tolkowsky’s and a number of steep/deep combinations where the diamond will appear smaller than its carat weight implies, an also may result in significant light leakage. Additionally, GIA rounds some of the measurements on grading reports. For this reason it is desirable to have a Helium or Sarin report for any GIA diamond, as it allows greater accuracy.
The GIA enjoys a global reputation for strictness in color, clarity and finish grading but allows more latitude in their grading of cut.
The American Gem Society (AGS) based cut grading on performance.
The AGS grades on a scale of 0 through 10. Diamonds earning the top grade of 0 are considered “Ideal.” Only a fraction of the world’s diamonds can earn this grade. In 2005 the AGS introduced the industry's first scientific cut grade based on light performance, allowing cut grading for shapes other than round. The AGS uses human analysis, angular spectrum (ASET) and a diamond-specific ray-tracing engine to determine the quantity and quality of light being returned to the viewer, in motion as well as in the face-up static position. The AGS 0 ‘Ideal’ grade is the world’s strictest laboratory standard for cut.
Q: Why are AGS-Graded diamonds so hard to find?
Simple. Over 97% of all diamonds fail to meet AGS 0 ‘Ideal’ requirements. This is why you’ll find commercial diamonds sent to labs with softer cut grading, or no cut grade at all. We have evaluated the different systems and find the AGS Lab’s cut grading methods to be the best , confirming our experience analyzing thousands of diamonds over many years.
>>Next: The 5th C - Confirmation