AGSL Princess Cut Grading – Summary

The following is an abbreviated version of the full article written in collaboration with the American Gem Society Laboratories (AGSL).  For a more in-depth look at this topic please see the full article.
To understand the peer reviewed science behind the AGS system, you can refer to the landmark study which appeared the 2007 in the journal Optical Engineering, entitled “Evaluation of Brilliance, Fire, and Scintillation in Round Brilliant Gemstones”.
Vatche 1504 Alegria Pave Diamond Engagement Ring
Vatche 1504 Alegria Pave Diamond Engagement Ring

The Princess Cut – Short History, Great Success

The princess is a relatively recent addition to the menu of diamond shapes, having been first introduced in the 1960’s. Development of the princess cut created a fusion between the sparkle of the round brilliant and the square to rectangular outline of step cuts such as the emerald and Asscher cuts. It is generically referred to as a square or rectangular modified brilliant cut and encompasses a variety of similar faceting patterns. Soon after its development the princess experienced a rapid rise in popularity to become the second leading diamond shape next to the classic round brilliant.

Facet Arrangements of the Princess Cut

The princess cut can have a variable number of facets. The crown can be arranged with or without bezel facets and the pavilion can feature a varying number of “chevron” facets.
There are two sets of main crown facets and two sets of main pavilion facets (as opposed to one of each for rounds), referred to as C1/C2 and P1/P2 in the images below. In addition to the main pavilion facets, variations of the princess include pavilion configurations of 2, 3, or 4 tiers of chevron facets. Larger stones are sometimes cut with even more tiers of chevrons as the greater surface area can support more facets without the facets getting too small to produce some broader, eye-catching sparkles.
Princess Cut Crown Diagram
Princess Cut Crown Diagram
Princess Cut Pavilion Diagram
Princess Cut Pavilion Diagram

Better Yield from the Rough

Because its overall shape mimics the shape of the octahedron, a common crystal form of gem diamond, the princess cut provides a way for diamond cutters to extract more finished diamond weight compared to rounds. Higher yields mean lower per carat costs resulting in lower price point options for the consumer- another reason for the popularity of the princess cut in the market.

Background on Cut Grading

AGSL is the only laboratory providing light performance cut grading on princess cuts. The GIA has long stated that they are developing a system, but as of this time they still do not provide an overall cut grade on princess.
Scientific cut quality analysis of the princess cut came about with the launch of the AGS light performance based cut grading system in May 2005. It was first released for round brilliant cuts and later expanded for the greater complexity of fancy shapes. The princess cut was the first of several shapes to be subsequently added to the system, and was released in June of 2005.
AGSL developed a methodology utilizing computer modeling and ray tracing technology to perform a detailed analysis of a diamond’s light handling ability. The test involves evaluating a 3D model of the diamond derived from one of several highly accurate non-contact measuring devices widely available in the diamond trade. The device measures each and every facet including size, angle and direction. Since light rays behave predictably according to the laws of physics, it is possible to calculate mathematically the various aspects of light performance. In this process 40,000 virtual light rays are applied to the model and results calculated. Simultaneously the proportion factors are determined. The final part of the cut analysis is done by trained graders who evaluate polish and meet-point symmetry. In all, the AGS Cut grading system analyzes eleven separate aspects of diamond performance and craftsmanship including light performance, proportions, and finish components:
  • Brightness
  • Leakage
  • Dispersion (fire)
  • Contrast
  • Girdle
  • Durability
  • Weight Ratio
  • Culet
  • Tilt
  • Polish
  • Symmetry
The AGS system grades on a 0-10 scale where 0 is Ideal. A deduction of any factor of more that 0.5, or cumulative deductions totaling more than 0.5, will prevent a diamond from achieving the AGS0 pedigree.

Princess ASET Maps

The graphical basis for the AGS system is called the Angular Spectrum Evaluation Tool or ASET (pronounced like the word “asset”) providing a color coded view of light return, light leakage and contrast.  In ASET red is light being drawn from higher angles overhead (between 45 and 75 degrees) where the brightest light is usually coming direct from the source.  Green is light drawn at lower angles (0-45 degrees) usually being reflected off objects surrounding the viewer.  Blue represents the light coming from the very highest angle and typically blocked by the viewers head (75 to 90 degrees).  Leakage is represented either by white or black, depending on the background.
Precision cut round diamonds tend to have a very similar ASET signature.  ASET views of princess cuts, perhaps not surprisingly given their range of facet designs, are more varied. But like rounds, well cut princess will have a mix of red light and well distributed green.  They will typically have some blue and a minimal amount of leakage.
Below are examples of different ASET images showing some of the variety seen in Ideal princess cuts.
Princess Cut ASET Images
Princess Cut ASET Images

A Word about Measurements

From the dimensions used to communicate princess measurements one can get a somewhat distorted sense of visual size relative to rounds.  The depth percentage of the princess is calculated as a ratio of the smallest width to total depth.  As a result we commonly see well made princess cuts with depth percentages in the mid 70’s as opposed to rounds in the neighborhood of 60%.  But if princess depth percentage were calculated differently, such as using a point to point measurement instead of smallest width, we would see depth percentages much smaller.
In terms of visual footprint, a well cut princess will appear slightly smaller than a comparable weight in a well cut round.  A 1.00 carat ideal cut round is approximately 6.5mm in diameter.  A 1.00 carat ideal princess is approximately 5.5 millimeter square.  Calculating the areas of these two shapes (circle= ∏ x R2 , rectangle= length x width)     the round has a visual footprint of 33.18 mm2 and the princess 30.25 mm2.  The round diamond is therefore about 10% larger in visual impact.  Another way to look at it is that in order for a princess to have a similar visual size to a 6.5mm round, it needs to be about 5.75mm square.
The princess cut offers a compelling value proposition because the price differential in the market between round and princess is much greater than the visual size difference.  This enables a shopper to get a princess of larger carat weight that is comparable to a round in visual impact, while still saving money.

Ideal Princess Cuts - "Rules of Thumb"

Certain generalizations can be made about high performing princess cuts which can be helpful when shopping.  These are only guidelines, not hard and fast rules.

  • The AGS grading system favors full crowns. A princess will usually not receive a 0 grade if the crown height is less than 12%. Fuller crowns in princess cuts produce more fire.
  • Smaller tables are favorable. It is very difficult to get of AGS Ideal when table size is 69% or greater.
  • Most ideals have a depth between 74.0% to 77.0%. (However it is still possible to achieve 0 outside this range)
  • As a general rule the table size should not exceed the depth percentage.
  • The AGS light performance system is designed for princess cuts with length to width ratios between 1.00 and 1.05

Market Perspective

There was no cut grade standard for princess cuts prior to the introduction of the AGSL light performance system in 2005.  GIA still has not released a cut system for any fancy shape diamonds. As a result, princess cuts where traditionally cut with the primary goal of retaining weight.  Cutting for beauty was secondary to getting the most carat weight possible out of the rough, in part because the technology to fully understand  their optics had not been invented yet.  Because of the dominance of GIA, the broader market is still very much the same today.  There are only a few diamond  manufacturers specializing in advanced cutting of princess cuts.

Because most diamonds are certified by GIA, and because they haver so far elected not to provide an overall cut grade on princess cuts, there is little incentive for cutters to change their manufacturing practices or marketing strategy. The result is a market dominated by mediocre princess cuts that fail to express the full potential for the beauty that most consumers value in a diamond.

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