Diamond Cut - The Most Important Of The 4 C's

There are two approaches used to analyze diamond cut quality. Traditional proportion assessment utilizes a 2 dimensional approach that takes the outer measurements of a diamond and predicts how it will handle light. Modern performance assessment measures the actual light behavior of the diamond and compares it to established standards.
In this article we will discuss both, but focus on the latter as it is the way of the future.
Diamond Proportions and Performance

Laboratory Cut Grading – Different Approaches

The Gemological Institute of America (GIA) uses a proportion based system which provides an overall cut grade for 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 cut grade is rendered by comparing measurements to predefined grading tables based upon GIA’s extensive research.
The American Gem Society Laboratories (AGSL) grades the cut of round diamonds 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). Light performance is calculated mathematically by ray tracing a 3 dimensional model of the diamond and taking into account the contribution of every facet.
You might say that proportions assessment looks at the container and performance assessment looks at the contents. Because the AGS cut grading system is much more sophisticated and less forgiving, consumers looking for the finest cut craftsmanship prefer AGS reports when available.

Proportions Assessment

A diamond has facets that allow light to enter, become refracted and exit in a rainbow of colors. When combined the light appears as white sparkles and when dispersed into its component colors appears as colored sparkles. Certain angles and measurements are proven to optimize these effects. The facet arrangement of the standard round brilliant diamond permits reliable proportion-based analysis by knowing a few key proportions. However, since these measurements are the result of averaging and rounding, deviations in a diamonds faceting precision are not revealed by this method of analysis.
Fancy shape diamonds are more complex and cannot be as accurately predicted using proportions alone.
Prism and Diamond

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 Facet Map
Round Brilliant Pavilion and Crown Views
Round Brilliant Diamond Facet Map
Light performance based cut grading such as computerized ray tracing of a 3D scan of the diamond as done by AGSL, takes into account the contribution of every facet to overall performance. Proportion based systems look at a limited number of averaged and rounded measurements.

The Tolkowsky Ideal

In 1919 a mathematician and diamond cutter named Marcel Tolkowsky published a book called Diamond Design wherein he mathematically calculated a set of proportions for the ‘Ideal’ round brilliant.
Tolkowsky Ideal
Tolkowsky’s observations of diamonds cut to these proportions, as well as the observations of others in the trade, served to validate his calculations and his work was widely embraced. His measurements became a standard by which the best cut diamonds were judged, and ultimately formed the basis for the first laboratory cut grading system – AGS Laboratories in 1996. As laboratories develop new systems we know that other beautiful configurations are possible, but Tolkowsky’s work has been largely validated by modern science.
It is important to note that although Tolkowsky is considered the ‘father of the ideal cut’, several visionary American diamond cutters were advocating very nearly the same proportions decades earlier. The story of these remarkable pioneers is wonderfully chronicled in Al Gibertson’s book “American Cut – The First 100 Years”.

Proportions Charts

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 Laboratories (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 (2005), with cut grading extended to certain fancy shapes as well. AGS proportion charts are still used as a reference for round brilliants where limited information is available.

AGS Traditional Ideal Proportions, 1996-2005

Round diamonds with overall measurements in this range are considered to have traditional ‘ideal’ proportions. As mentioned above, in 2005 AGS transitioned to ray tracing performance assessment, and diamonds at the outer limits of this traditional range may no longer receive their top grade. Faceting precision (3D facet alignment) will also impact the final grade.
AGS Ideal Proportions

Super Ideal Proportions

Super Ideals are precision cut diamonds targeting the sweet spot or ‘bulls-eye” of traditional ideal proportions.
Stricter than traditional AGS proportions, above are the Whiteflash proportion guidelines for A CUT ABOVE® Hearts & Arrows diamonds. Round brilliant diamonds cut with precision in this range of measurements receive the highest ratings in both proportions and performance grading systems.
Whiteflash Criteria
A CUT ABOVE® diamonds must AGSL light performance grade of Ideal, exhibit perfect hearts and arrows patterning and pass multiple additional tests and evaluations by the Whiteflash review team. For a full understanding of these elite diamonds, please see our specifications and qualifications page.
*A CUT ABOVE® super ideal diamonds in both round and princess cut are available only through Whiteflash and are always in-stock with a comprehensive set of data and imagery posted to the website.

Online Cut Estimators

Another type of proportions assessment involves plugging measurements into a cut estimator for a computer assisted prediction of performance.

Holloway Cut Adviser (HCA)

In the 1990s an Australian geologist named Garry Holloway introduced the HCA tool, a free online cut analyzer that gained significant popularity and is still in use today. Analyzing 5 basic measurements, it is possible to get an indication of predicted performance. It outputs a numerical result (2 or less is considered the top range), along with a text based descriptions of brightness, fire and spread.
As a two dimensional tool capable of assessing only a limited number of facets, the HCA is of no value when 3 dimensional analyses such as an AGS light performance report, or when light performance imagery such as ASET or Ideal-Scope is available. But it can be used effectively as s filter to eliminate problematic proportion combinations from a pool of diamonds such as virtual inventories found online.

GIA Facetware

In 2006 GIA introduced their cut grading system together with a similar free online cut estimator. The Facetware tool assesses additional information about the diamond, but is still essentially a proportion based system. The input data is compared to pre-defined grade tables to render an estimate of the cut grade it would likely receive at the GIA lab.

Light Performance Assessment

Where proportions assessment is a prediction based on measurements, performance assessment looks at the actual light behavior of a diamond. Diamond performance has traditionally been described in terms of brilliance, fire and scintillation.
Brilliance or Brightness essentially refers to quantity of light returning from the diamond. Fire (technically known as dispersion), is the degree to which the white light is broken out or “dispersed” into spectral colors. Scintillation is sparkle - a dynamic effect associated with movement requiring positive contrast for an ‘on/off’ blinking effect.
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 overall quantity and quality of light output of a diamond and compare it to established standards.
The symmetrical facet arrangement of the round brilliant diamond permits meaningful analysis by knowing some basic proportions, but fancy shapes are more complex and cannot be decisively predicted in this manner. Light performance analysis is valuable for all diamonds but particularly useful for fancy shapes. Since GIA only provides an overall cut grade on rounds, it is particularly important to have additional diagnostics to fully understand the cut quality of fancies.

AGS Laboratory Cut Grading

The most sophisticated and stringent cut grading system in use today is scientifically vetted the light performance system in use at AGSL. Each diamond is scanned and individually modeled and 40,000 virtual light rays are traced at a variety of tilt angles to measure performance aspects of brightness, dispersion (fire), contrast and light leakage.
AGS Light Performance Grading Hemisphere
AGS Light Performance Grading Hemisphere
For an overview of the science and process please see our article on AGSL Cut Grading. In addition to round diamonds AGSL also performs cut grading on fancy shapes. For a detailed look at grading of the most popular fancy shape please see Princess Cut Grading at AGSL.

Computer Simulation

Sophisticated fee-based tools are used by some diamond cutters, researchers, and gemologists using a digital scan of the diamond (such as a Sarine file) to analyze a 3D 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.
DiamCalc screen set to wire frame and basic ray tracing
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.

Reflector Tools

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.

Ideal-Scope

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.
Ideal Scope Viewer

ASET

In 2005 the AGS introduced a similar tool, the Angular Spectrum Evaluation Tool or ASET (pronounced ‘asset’). 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). An ASET image therefore provides more specific information than Ideal-Scope about how the diamond is handling light.
ASET

Hearts and Arrows Viewer

Patterning resulting from different levels of faceting design and precision can impact light performance and diamond beauty in significant ways. Well-proportioned diamonds cut to a very high level of 3 dimensional symmetry (also referred to as optical symmetry or optical precision), exhibit a unique pattern of hearts (face down - pavilion view) and arrows (face up - table view). This Hearts and Arrows pattern (H&A) is visible under a special reflector device.
Hearts and Arrows Viewer
An accurate hearts and arrows pattern indicates that corresponding facets are lined up precisely in three dimensions such that their light handling capabilities are optimized. Diamonds with H&A pattern can still have light performance deficits that can be detected by ASET and Ideal Scope. The combination of ideal proportions with optical precision maximizes the benefits of hearts and arrows patterning.

Other Direct Assessment Devices

A number of other devices have been developed to attempt to measure and analyze a diamond’s light output. ImaGem and BrillianceScope are among the most well-known. These machines provide interesting demonstrations and information that may be useful, but each gives different results. Some of these technologies have been adopted by chain jewelry stores for use in diamond presentations.
We have tested a number of automated/computerized light output measurement 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 in the future if it can be proven to be repeatable and provide results consistent with scientifically vetted grading systems such as AGSL.

Human Assessment

Mathematical estimators, reflector images, and even sophisticated ray tracing technologies each have their place can be very useful and. But diamonds are dynamic and the perception of beauty is complex, and ultimately subjective. We don’t all respond the same way to the same visual stimuli. Ultimately the human eye is the best tool of all.
For someone not experienced in looking at diamonds, consulting a trained independent professional is a very good idea. Certain diamond characteristics are very subtle and hard for the untrained eye to appreciate. But subtle aspects can impact performance in meaningful ways, so expert advice is the best way to make sure you are getting everything you bargained for in your diamond purchase.

Conclusions

If you’re shopping on the internet, finding diamonds with traditional ideal proportions is a fast way to locate good candidates. Online cut estimators can serves as preliminary filters. Then you can use reflector images and other performance data to see which has the best light return and patterning. Diamonds that have been ray traced, such as AGSL certified stones, provide an even higher level of understanding. But human observation is always the final arbiter of beauty and overall value in a purchase. The best of all worlds is to have all the diagnostics for a full technical understanding and documentation of the diamond, and visual confirmation of its beauty.
The amount of information available about a given diamond varies dramatically in the market. Some merchants offer very limited analysis while others specialize in providing complete evaluations including images, videos, and light performance lab reports.
A very positive by-product of internet e-commerce is that consumers are becoming better educated about diamond quality and how to assess it for themselves. When shopping in retail stores you should seek the same data, imagery, and documentation that you would expect on the internet.

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