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The GIA Excellent Cut Grade

Some content on this page are contributed by Good Old Gold, with thanks.

The science behind GIA's cut grading system was ultimately determined by human observation testing and is perhaps the greatest strength of their system.  The basis for the GIA Cut Grading system is how a person sees, in the most common lighting environments, the optical characteristics of brightness (and implicitly contrast), fire and scintillation.  Any stone that displays a decrease in any of these optical elements contributes to a lower cut grade.

GIA's study took place over the course of 15 years. At the beginning of their studies they did utilise technologies, the primary one being that of ray tracing software with which they could build a virtual diamond of any proportions and track how light behaves in such a stone.  Over the last 5 years of their study the focus of their research was in conducting observation testing with people in the trade as well as consumers.  GIA created a lighting environment which duplicates the lighting conditions which most people find themselves in on a daily basis. In this controlled environment, they can observe the brightness, fire & scintillation which is captured in a large database of human observations.

Tools and Methodology

The primary tool is GIA's DiamondDock.  This is a very simple contraption. It is essentially a box that has neutral gray walls providing 2 types of lighting conditions. This is necessary to ensure that all the human observation results captured in their database were collected under the same lighting conditions.

The diamonds were all observed in a neutral grey backdrop (tray) from a stipulated distance.

The 2 specially calibrated lighting conditions provided by the DiamondDock are as follows:

  1. daylight fluorescence, to observe brightness & patterned scintillation. 
  2. 5800 degree kelvin l.e.d. (light emitting diodes) to observe fire and sparkle scintillation.

We can turn on both fluorescent and l.e.d.'s and observe a nice mix of both brightness, fire and scintillation (both patterned and static).

After their observation testing was complete, GIA also developed the GIA FacetWare Software which is available for free online at this link.

Other than the GIA FacetWare results, the diamond must also have a minimum of very good polish, very good symmetry and none to minimal painting and digging on the girdle facets in order to qualify for the GIA Excellent cut grade.

What It Does Well

A diamond that receives the GIA Excellent Cut Grade will be a bright and beautiful diamond and this was determined by those who did the viewing (with over 70,000 observations on more than 2300 diamonds.).

The FacetWare program is based on approx. 38.5 million proportion combinations in which "face up appearance" is determined. So, it is very comprehensive.


Every system has its limitations, and the GIA cut grading system is no exception. We list some of its flaws as follows:

Limitations 1: The Steep/Deep GIA Excellent

As the system is based on observation testing and the criteria set are mainly brightness, fire and scintillation as seen by people, any diamond that meets the criteria will be graded as Excellent. Unfortunately this set of criteria would qualify some diamonds that have crowns that are too steep and pavilions that are too deep (which we shall call a steep/deep diamond) as Excellent. For example, a diamond with the following proportions would qualify as Excellent:

A steep/deep diamond can be visually beautiful if the other facets are cut to proportion. This is because the light leakage forms a ring under table, which we refer to as the 'ring-of-death'. However, if the light return from the rest of the diamond is good, this ring will be visually obscured because it is bounded by areas of good light return.

The following diamond, which we used earlier to illustrate the flaws of the old AGS Triple 0 Ideal grade would make it as a GIA Excellent:

1.238ct G VS2

There is nothing seriously wrong with a steep/deep GIA Excellent because it is visually good looking. However, we must remember that a steep/deep diamond carries extra weight that does nothing except to decrease the performance of the diamond. This is no laughing matter when you have to pay for the extra weight.

This shows that there are some critical elements to cut grading that observation testing does not see or anlayse.

Limitations 2: No Optical Symmetry Grade

The GIA cut grading system does not grade the optical symmetry (not to be confused with the cut symmetry) of a diamond. Optical symmetry deals with the quality of the observable cut pattern of a diamond, particularly the Hearts and Arrows Pattern. It does not provide any information on the cut pattern to help consumers make their buying decision either.

The importance of optical symmetry can be found in discussions on scintillation and the key minor facets.

Consumers have to rely on a trustworthy vendor to provide such information if optical symmetry is important to them.

Limitations 3: No Information on Girdle Digging and Painting

Although GIA does a good job at weeding out diamonds with girdle digging and painting, it does allow some girdle painting in its Excellent cut grade. This is because a diamond with some girdle painting may not look bad to most people. So, it would make it pass GIA's observation testing.

However, whether a person likes a diamond with painted girdle or not is a matter of taste. So, it would be more prudent for a consumer to obtain such information from the vendor before making a purchase.

In Summary

The GIA cut grading system is an excellent system and a great boon for consumers. We buy diamonds because they look good, and the GIA cut grading system ensures that it does, through its observation testing.

Never-the-less, every system has its limitations. A consumer has to be wary of the pitfalls of the cut grade and not rely on it blindly.




Next: The AGS Ideal...