We urge you to read this section carefully. An investment of time here will give you a knowledge advantage which only a fraction of shoppers enjoy.
Cut is absolutely, unarguably the most important C for diamond beauty. It is also the most difficult to understand.
Diamonds in the top few percent of the world’s cut actually improve in the other Cs: They appear larger for their carat weight, face up more colorless and clarity characteristics can be less visible. Most importantly, they explode with dazzling whiteness and rainbow colors, even away from jewelry store lighting where most diamonds go dark. Even average cut quality makes a diamond flash and sparkle in bright lighting, though it will not enjoy benefits to the other Cs. Poor cut quality may look attractive under bright lights but in normal lighting can make a diamond appear small, tinted and lifeless.
Common Cut Standards
Despite its preeminent importance cut is the least discussed C in most markets. Why? One reason is that color and clarity grading are easily learned, but understanding light performance within diamonds requires some geometry and physics. Some professionals seek education but the vast majority of workday salespeople are not trained and do not pursue it. It’s worth noting that most diamonds are cut to average quality and in-depth discussions about cut would reveal that lack of quality in average stores. The fact that many grading labs include little or no information about cut only underscores the resistance to cut education felt in commercial markets.
AGS and GIA Cut Standards
The American Gem Society (AGS) and Gemological Institute of America (GIA) are the leaders at enforcing standards for cut performance (although neither grades cut precision).
The American Gem Society (AGS) boasts the only cut grading system recognized by the science community. Published by SPIE, the International Society for Optical Engineering, theirs is a diamond-specific metric which evaluates angular spectrum in three-dimensions and takes different distances and tilt into account. Fewer than 1 in 20 round diamonds are capable of earning the AGS Ideal grade and this number falls to 1 in 100 for princess cuts. Only 2% of diamonds are sent to the AGS for grading, so diamonds graded there are rare.
The GIA employs a two-dimensional cut metric for round diamonds. Approximately the best 15% of round diamonds will earn the top grade in the GIA system, although some fall in a steep/deep area that does not result in high performance. The GIA does not yet grade cut fancy shapes.
Going Further: Performance and Precision
The cut grades issued by the AGS and GIA grade cut performance, but not cut precision.
This is how well light gets in and out of a diamond. The components of performance are brightness (all light returning to the eye), dispersion (“fire” seen as white light is broken into its spectral colours) contrast (the pattern of dark areas which exaggerates light areas) and scintillation (sparkle seen as the diamond, the light source or the observer move). The benefits of high performance are maximized light return for higher values in all these categories.
Performance has been judged in diamonds for many years. The level of performance, particularly brightness and contrast, can be seen in the AGS ASET® and its predecessor the Ideal-Scope®.
Round Brilliant Examples in ASET® and Ideal-Scope®
This is how well the diamond’s facets (mirrors) align with each other in 3D. Precision components include optical symmetry (uniformity of patterning, such as the “Hearts & Arrows” pattern seen in precise round brilliants) and the degree of crispness of those patterns. The benefits of high precision are increased contrast values, purer spectral colours during scintillation and improved performance in low-light conditions.
Precision is a young aspect of cut that became possible in the 1980s and 1990s in rounds and is only now surfacing in fancy shapes. Achieving top precision requires the most modern tools, extra time and higher skill sets to accomplish. The level of precision can be seen by using an Optical Symmetry or “Hearts & Arrows” viewer.
Round Brilliant Examples in Optical Symmetry Viewer
Diamonds With Both Performance and Precision
Only a fraction of the world’s diamonds have both top performance and top precision and are quite rare. Such diamonds actually improve in the other Cs: They appear larger for their carat weight, face up more colourless and clarity characteristics can be less visible. They also have dazzling whiteness and rainbow colours, even away from the jewelry store lighting.
It is important to remember that labs do not judge both aspects of cut: A diamond can have great performance without enjoying the benefits of precision. Others can be very precisely cut without the benefits of top performance.