Perfect Proportion

RAPAPORT... The American Gem Society (AGS) Laboratories has introduced a newly developed grading report based on its advanced light performance methodology. The objective of the proportion-based Diamond Quality™ Report is to provide a simpler grading product that manufacturers and retailers can easily understand and use.

The new report is adapted from AGS’s patented Light Performance System that measures how well a diamond has been cut.

The Diamond Quality Report is designed to provide operational and marketing advantages. Operationally, the cutting instructions are on one page and there are 14 cutting charts designed to help manufacturers cut rough to achieve the best cuts and retain weight.

The second advantage falls in the marketing realm. The new AGS Laboratories report uses six word descriptors for a diamond’s cut grade: Ideal, Excellent, Very Good, Good, Fair and Poor. AGS Laboratories-graded diamonds typically carry a market price premium of 3 to 5 percent, according to Frank Dallahan, chief executive officer (CEO).

Background
Although the AGS’s Performance Grading Software analyzes a diamond’s cut quality in three dimensions, the lab received requests from both manufacturing and retail segments of the industry for a simpler and easier-to-understand cut grading system for the standard round brilliant cut. The new report is based on two dimensions while the light performance report is based on a three-dimensional system.

Reasons Why
There are a number of good reasons to develop a proportion-type cut grading system.
• AGS invented the first proportion system and made it part of its Diamond Grading Standards in the 1960s.
• Until the advent of the AGS Light Performance System, most, if not all, cut grading systems used proportions.
• All tiers of the industry are comfortable working with proportions, angles and percentages.
• Gemologists are trained using proportions.
• Diamond cutters work with angles every day in their profession.
• This type of system provides manufacturers with 100 percent predictability in planning and cutting diamond rough.
• Proportion grading is much simpler to learn, understand, teach and explain.

Methodology
AGS Laboratories used the AGS’s patented Performance Grading Software to examine all of the proportion sets included in the new grading system. Instead of merely assessing the face-up appearance of each proportion set, a very large database was created to examine each proportion set over a range of tilts from face-up through 35 degrees in one-degree increments. Each proportion set was ray traced a total of 36 times to reflect the fact that when people view a diamond, they typically tilt it forward and back.

Using Light Performance map imagery, Figure 1, at right, shows, visually, the amount of analysis that went into each proportion set.

During the creation of these images, the software simultaneously collects and tallies all of the values for brightness, contrast, dispersion and leakage. The face-up values are given 50 percent of the grade and the average of the 35 tilts make up the other 50 percent of the grade.

Previous proportion-based grading systems only addressed the face-up position. The benefit of the new approach — using multiple tilts — is that it also accounts for and takes into consideration how well a diamond performs over a range of tilts. This depth of analysis is a huge leap forward in proportion analysis.

In addition, multiple databases were created that measured each proportion set’s performance with varying star facet lengths, lower girdle facet lengths, girdle thicknesses and performance variations caused by the “painting” and “digging” of the upper and lower half facets.

Making it Simple
Rather than grading every conceivable way to cut a round brilliant, the table diameter percentage range of 52 through 65 was chosen because this is where the vast majority of round brilliants are cut. Reasonable and effective parameters for girdle thickness were set based on the same rationale. In a proportion-based system, girdle thickness also helps in determining realistic weight ratio parameters. Ranges for star length, lower girdle length, “painting” and “digging” were established as well. The use of ranges helped simplify the system. Culet size parameters also were implemented.

Finish – polish and symmetry – parameters were also established that reflect craftsmanship while remaining practical for the manufacturing industry.
Lastly, averaging and rounding parameters were defined. In any proportion-based grading system, averaging and rounding are required or the system would balloon into a confusing array of parameters, charts and rules.

The Grades
The AGS Light Performance System uses the traditional grades of AGS Ideal 0 through AGS Poor 10, which the proportion system simplifies to Ideal, Excellent, Very Good, Good, Fair and Poor.

Another goal was to provide a realistic distribution of grades to reflect the fact that there are more Excellent cuts than Ideal cuts, more Very Good cuts than Excellent cuts, more Good cuts than Very Good cuts, etc.
Figure 2 summarizes the distribution of grades.

The System:

Polish and Symmetry
• The Ideal Cut Grade can have Ideal or Excellent polish and symmetry.
• An Excellent Cut Grade can have Ideal or Excellent or Very Good polish and symmetry.
• Essentially, polish and symmetry grades can be one grade lower than the assigned cut grade.
Table Diameter Sizes
•52 to 65 percent.

Girdle Thickness
• The Ideal Cut Grade can have girdle thicknesses of Thin, Medium or Slightly Thick at the “valleys” or “scallops.”
• Highest grade achievable with up to a 4.5 percent girdle, at any point, is Ideal.
• Highest grade achievable with >4.5 to 5.5 percent girdle, at any point, is Excellent.
• Highest grade achievable with >5.5 to 6.5 percent girdle, at any point, is Very Good.
• Highest grade achievable with >6.5 to 7.5 percent girdle, at any point, is Good.
• Highest grade achievable with >7.5 to 10.0 percent girdle, at any point, is Fair.
• Highest grade achievable with >10.0 percent girdle, at any point, is Poor.
• Extremely thin or very thin girdle, at any point, reduces the cut grade by one grade.

Weight Ratio
• This is factored into the Proportion Charts.
Star Length
• All inclusive from 45 to 55 percent.
Lower Girdle Length
• All inclusive from 75 to 85 percent.
Painting and Digging
• Maximum digging of the upper half facets is 3 degrees of azimuth
• Maximum painting of the upper half facets is 4 degrees of azimuth.
• Maximum digging of the lower half facets is 3 degrees of azimuth.
• Maximum painting of the lower half facets is 3 degrees of azimuth.

Rounding
• Table diameter percentage is rounded to the nearest 1 percent: 55.4 percent = 55 percent, 55.5 percent = 56 percent.
• Pavilion angle is rounded to the nearest tenth of a degree: 40.74 degrees = 40.7 degrees, 40.75 degrees = 40.8 degrees.
• Crown angle is rounded to the nearest half degree: 34.24 degrees = 34 degrees, 34.25 degrees = 34.5 degrees.
Note: All angles and percentages on documents are reported to the accuracy of the Sarin DiaMension™ measuring device, but the above rounding is used for calculating the proportion cut grade.

Culet Size
• Highest grade achievable with none, very small, small or medium is Ideal.
• Highest grade achievable with slightly large is Very Good.
• Highest grade achievable with large is Good.
• Highest grade achievable with very large or extremely large is Fair.
Any standard round brilliant cut outside of any of the above parameters must be cut-graded using the patented AGS Performance Grading Software.

Conclusion
The new proportion cut grading system provides all tiers of the industry with choices as to which type of cut grading they prefer — light performance or proportions.
Some of the benefits of the proportion-based system are:
• Practicality, consistency and feasibility in manufacturing.
• Ease in understanding a grading method.
• Ease in learning a grading method.
• Ease in teaching a grading method.
• Ease in communicating the importance of diamond cut to a consumer.
The proportion system can be downloaded here.

Article from the Rapaport Magazine - December 2008. To subscribe click here.