The 4 C's - Diamond Cut: Cut Grading and Buying Guide

Cut quality is the only diamond value factor that is the result of human input. This vital factor may be the most difficult to understand because it is the most technical, but gaining a basic understanding of diamond cut is important to making a wise buying decision. Cut refers to the proportioning, alignment and finish of a diamond's polished faces or facets. Proper cutting enables a diamond to perform at its best and release its full potential for fire and brilliance. A well cut diamond will be more beautiful and more valuable than a diamond whose cut quality has been compromised for carat weight.
*Note: The word "cut" is also used in reference to a diamond's shape - round brilliant cut, marquise cut, princess cut, etc. It is less confusing to restrict the term "cut" to a discussion of the proportions, facet alignment and finish of a polished diamond.
A diamond cutter must spend more time and sacrifice more of the rough crystal in order to produce a well cut finished stone, thereby ending up with a smaller diamond than had he taken liberties with cut quality. Well cut diamonds therefore yield less carat weight from the rough and are consequently more expensive to produce.
The importance of diamond cut cannot be over emphasized. The consensus view of virtually every diamond expert can be summed up in the words on the website of the GIA (Gemological Institute of America). “The allure of a particular diamond depends more on cut than anything else.”

Anatomy of a Diamond

The following illustrations show the various parts of a diamond and measurements used in the analysis of a diamond’s cut:
Diamond Anatomy
Image courtesy of AGS Laboratories

Light Return and Light Leakage

Below is a depiction of the basic behavior of light entering a diamond. A well cut diamond will refract and reflect a high percentage of light back to the eye, whereas a poorly cut diamond will allow light to leak out the bottom resulting in diminished brilliance and fire.
Diamond Cuts

Light Performance, Proportions and Finish

The AGS Laboratories perform the most comprehensive and critical cut quality analysis of any lab in the world. It is therefore instructive to understand the range of factors taken into account by this rigorous and scientifically vetted cut grading system. There are eleven individual factors evaluated:
Light Performance Factors
  • Brightness
  • Dispersion (fire)
  • Leakage
  • Contrast
Proportions Factors
  • Girdle Thickness
  • Culet Size
  • Weight Ratio (spread)
  • Durability
  • Tilt
Finish Factors
  • Polish
  • Symmetry
The AGSL Light Performance grading system evaluates every facet of the diamond, both major and minor, in arriving at a cut grade for each aspect. For a detailed explanation please see our article entitled AGS Cut Grading. The article is part of a comprehensive series written in collaboration with the American Gem Society Laboratories that also includes light performance based cut grading of Princess diamonds.

Fire, Brilliance, and Scintillation

The three aspects of diamond beauty are generally referred to as fire, brilliance and scintillation (sparkle). They are distinct but inter-related characteristics. For a diamond to express its full potential and to be as beautiful as possible, it must be crafted such that these characteristics are optimized in a balanced way.
Fire refers to the ability of a faceted diamond to act as a prism and to break white light into the individual colors of the visible spectrum. Under lighting conditions conducive to the observation of fire, a well cut diamond will exhibit “sparks” of red, green, yellow, and blue in the process of the light interacting with each of the diamond’s facets and being returned to the eye.
Diamond Fire and Scintillation
Diamond Fire and Scintillation
Brilliance is generally thought of as the total amount of light returned to the eye resulting in the overall brightness of the stone.
Scintillation is the sparkle of the diamond created by the dynamic “on-off” pattern of blinking facets in response to movement. In order to have maximum sparkle the cut of a diamond has to be designed and executed to create a balance of brightness and positive contrast.
The quality of a diamond's cut is a complicated interaction of proportions, facet angles, and finishing factors. Many combinations can result in beautiful finished stones. Because the above mentioned performance elements are distinct and interrelated, it is possible for the cutter to make tradeoffs in the design process. For instance, making the table facet a little larger may make the stone slightly more brilliant and slightly less fiery. So, even amongst the best diamond cuts there is still room for differences in personal preference and “taste”.

Ideal Cut Diamonds and the Labs

Diamonds of top cut quality are often referred to as Ideal Diamonds. Of the two most well respected laboratories only AGSL (American Gem Society Laboratories) actually certifies diamonds as Ideal. Their scientifically rigorous light performance based system analyzes eleven different aspects of cut quality and involves analysis of a 3D model of every facet of the diamond. This system enables AGSL to scientifically analyze and grade cut quality on round as well as princess diamonds, and several other shapes including oval, Asscher, emerald cut and cushion. Diamonds that earn the AGS Ideal grade are the “best of the best” in terms of cut quality.
The GIA cut grading system is broader and more forgiving and involves comparing a limited set of measurements to predetermined tables. “Excellent” is the highest grade in this system, and overall diamond cut grading is limited to round diamonds. (The cut related aspects of polish and symmetry are reported on all diamonds, but only rounds receive an overall cut grade at GIA). Because of the stature of GIA worldwide, there is a tendency for consumers to equate GIA Excellent with Ideal, despite the fact that a great number of GIA Excellent cut grades will fall well outside of the AGS Ideal category. Having said that, a diamond with a cut grade of Excellent from GIA will be very pretty (provided there are no clarity features that diminish light performance). For more information about differences in cut grades between the two labs please see the articles about AGS vs GIA grading and AGS0 vs GIA EX.
The concept of an "ideal" cut gained widespread notoriety in 1919 when mathematician Marcel Tolkowsky from Belgium published a study of the behavior of light in a cut diamond and came up with a formula for proportions that would result in the most beautiful round diamond. Prior to this time, American diamond cutters had also arrived at similar conclusions about proportions that resulted in the best optics. Modern science has tended to validate work of these pioneers who were going against the grain of a diamond industry that favored highest yield over best light performance.
The beauty of selecting an AGS Ideal diamond is that the guesswork and the need for calculating all the inter-related measurements and angles has been done for you. This allows you to focus attention on the other things that matter: color, clarity and size.

Diamond Analytics

In addition to the various tests performed at the laboratories in order to render a quality report, there are a variety of diagnostic tools available for assessing diamond craftsmanship and light performance that are available to the trade and to the consumer market. Reflector tools such as ASET and Ideal-Scope reveal details about how a diamond is handling light. Hearts and Arrows viewers give a visual picture of a diamond’s optical precision. Other online tools such as Holloway Cut Advisor (HCA) provide quick filters for spotting diamonds with poor proportions. Highly accurate non-contact measuring devices such as Sarine DiaScan provide detailed data and reports and generate 3D models of a diamond that can be used in sophisticated ray tracing programs such as Octonus DiamCalc.

Hearts and Arrows Diamonds

Hearts and Arrows diamonds or “H&A” diamonds are round brilliant diamonds that are cut very precisely in terms of their three dimensional or “optical” symmetry. Only when the cut craftsmanship is extremely precise will facets align in such a way that a pattern of eight hearts (pavilion view) and eight arrows (table view) will be visible under a special reflector device. It is important to understand that a diamond can have excellent H&A patterning without also having Ideal proportions or light performance. Diamonds that are both certified ideal with outstanding hearts and arrows patterning are referred to as Super Ideal Diamonds.
Hearts and Arrows Viewer
Hearts and Arrows Viewer
The benefits of hearts and arrows extend well beyond pleasing symmetrical patterns. When all facets are aligned precisely in three dimensions, the diamond is ‘tuned’ for optimal performance. For more details on H&A diamonds please see our articles hearts and arrows formation and grading.

Fancy Shape Diamonds

Diamond shapes other than round are referred to as “fancy” shapes. Almost any shape is possible and there are new designs coming into the market as new diamond cutting technologies and techniques are developed. Determining a cut grade for fancy shapes is far more complicated than for rounds, and as a result only the AGS light performance system is capable of accounting for the complexity. Even with this adaptive scientific system, not all shapes have yet been fully worked out.
One of the most important aspects in fancies is the overall outline of the shape, deriving primarily from its length to width ratio. The outline will affect the eye appeal of the diamond as well as the overall design of a finished piece of jewelry. To some extent this is a subjective factor allowing for different tastes. For instance, some people might prefer a long slender marquise for a design that accentuates a long slender finger. Likewise, a pendant design might favor an elongated pear shape over one with broad shoulders.
Whiteflash specializes in ideal cut round and princess diamonds, and normally does not stock other fancy shapes. However, our expertise in diamond light performance and our access to the global diamond network enables us to source top quality diamonds through our Virtual Diamond Service. Our GIA trained diamond consultants are highly experienced at guiding customers through a search for the diamond that is just right for their preferences and budget.
Below are guidelines that we use in our search for high performing fancy cut diamonds. Falling within these guidelines is not a guarantee that a diamond will be a top performer, but they serve as a good starting point for searching through databases of available inventory. Because performance of fancy cuts is much harder to predict from specifications alone, for the customer looking for a top performing fancy it is essential to have the diamond professionally inspected and to see light performance imagery before making a final decision.
Length to Width Ratio
Personal Preference

Summary - Practical Advice

Diamond cut is the number one factor determining a diamond’s beauty. Choosing an AGS Ideal is recommended for those looking for the ‘best of the best’ in terms of light performance. Choosing a GIA Excellent cut will assure you of very nice performance and in some cases performance at the top of the scale. In order to determine actual light performance on GIA diamonds, extra testing is required.

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