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GIA versus AGSL diamond grading reports

The Gemological Institute of America (GIA) and the American Gem Society Laboratory (AGSL) are recognized throughout the world as being a reputable and reliable source of diamond grading information. On a consumer level, they are most widely recognized for the diamond grading reports which they issue, but they also conduct in-depth gemological research which is heavily relied upon by the diamond industry, especially where it pertains to the ongoing research into the effects of diamond cut quality upon light return and visual performance.

In our experience the GIA and AGSL grade the basic characteristics represented by the 4C’s of Diamond Grading, such as Carat weight, Color, and Clarity, pretty much equal, given the understanding that each clarity and color grade is actually based upon a range, and as such there are bound to be differences of opinion from time to time, especially if the grade of the diamond lies on the outer edges of the range designated for that range of color or clarity.

How GIA and AGSL Grading Differs on Diamond Cut Quality:

However there is a distinct difference in how the two gemological laboratories determine the Overall Cut Grade of a diamond, while the GIA determines the cut grade of a diamond by taking polish, symmetry, and proportions into account, the AGSL uses a performance based system which takes those factors, plus the visual performance of the diamond into account.

The diamond grading platform currently relied upon by the AGSL, is the most thorough of the cut grade systems in use by any of the top tier gemological laboratories. It divides the overall cut grade into three primary categories, which take eleven factors of diamond cut quality into account:

  1. Light Performance:
  • Brightness
  • Dispersion
  • Leakage
  • Contrast
  1. Proportions
  • Durability
  • Tilt
  • Weight ratio
  • Girdle thickness
  • Culet size
  1. Finish
  • Polish
  • Symmetry

Angular Spectrum Evaluation Technology (ASET):

ASET - angular spectrum evaluation technologyIn 2005, the AGSL changed the way we look at diamonds and judge diamond cut quality, by introducing Angular Spectrum Evaluation Technology (ASET) via their proprietary Light Performance Grading platform. The ASET grading platform objectively evaluates nine of the eleven factors of diamond cut grading using a sophisticated system of optical ray tracing, the results of which are then translated into an image like this one that appears on the Diamond Quality Document (DQD) for this 3.087 carat, G-color, VS-2 clarity, Brian Gavin Signature diamond. The ASET image provides insight into the light performance of the diamond which is easily assimilated and understood on the consumer level.

What do the different colors on an ASET image mean?

The color red represents light which is striking the diamond from 45° up to perpendicular with the table facet, this is the brightest light which makes the diamond brilliant.

The color green represents light which is not as bright, it strikes the diamond from 45° out to the horizon, however note that it is completely normal for the center region of the table facet to be red, green or a combination thereof because they share the 45° factor.

The color blue represents contrast which is what enables our eyes to see, this is light which could have entered the diamond, but which is being blocked by our heads (or a camera lens) as we observe the diamond. The eight pointed star pattern which is visible in this photograph of a Brian Gavin Signature Hearts and Arrows Diamond is a reflection of the eight pavilion main facets which are part of the lower half of the diamond.

Black and White represents leakage which is present in all diamonds to some extent, whether the leakage appears as black or white depends on the lighting environment being used.

What to read next: Differences in ASET Images for High Performance Diamonds

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