by Don Clark CSM IMG
The Gemological Institute of America (GIA) grades the cut of a diamond as either;
- Excellent (the best grade)
- Very Good
- Poor (the worst grade)
Below are the details that will help you understand GIA's diamond cut grading system, and what distinguishes an "excellent" from a "good" or "fair" cut.
Excellent Grade (EX)
Excellent Grade has high brilliance, scintillation and an even pattern of light and dark areas.
Very Good Grade (VG)
This grade also has high brilliance and scintillation, but expect them to be a bit darker in the center or around the edges. In some cases a diamond can have top brilliance and scintillation, but be down graded by its pattern. The stone above has a splintery pattern that prevents it from being graded excellent.
Good Grade (G)
This grade will be generally be a bit darker or lacking in scintillation. The stone above is dark around its girdle. A stone can also be downgraded by its pattern, or its weight ratio. I.E. if it weighs more than an average gem of this diameter, it will be downgraded one step.
Fair Grade (F)
A gem in this category will have very little brilliance or scintillation. The stone in the picture has a very dark ring around the girdle and a dark table area.
Poor Grade (P)
Diamonds in this category have very poor proportions and show very little brilliance or scintillation. Expect to see "nail heads", where the diamond is black in the center and "fish eyes" where the brilliance washes out in the middle.
Many poor grades will have a decent appearance, but be downgraded by their weight ratio.
In the 1950's the Gemological Institute of America established a system for grading diamonds based on the four C's. This was a great asset to the diamond community and it became the standard for grading world wide. While the system for grading color and clarity removed most redundancies, the standards for grading the cut left something to be desired. That is not necessarily a criticism of the GIA; no one has been able to establish a simple a usable system that would cover all the variables involved.
The GIA recently released a new cut grading system and it appears they have finally conquered this beast. Rather than calculating how close a cut came to an ideal set of proportions, the new system is based on how the individual elements combine to affect the appearance of a diamond. In addition, they were able to install some flexibility in the system to allow for personal preferences, with table size being a prime example.
To accomplish this they had to develop a method to quantify beauty, determine how the different aspects of cutting affect the appearance of a diamond, develop a workable methodology to measure the parameters, and finally create a searchable database to make it easy to calculate the cut grade. The enormity of this task cannot be understated and they deserve great credit for their efforts.
Evaluating the Components of Cut
The new system is based on seven components; the first three are visual, the others physical.
- Scintillation The small areas of light in a polished diamond that flash on and off as the diamond, observer or lighting moves. There are also negative scintillation effects which include windowing, dark centers and dark upper girdles.
- Pattern The arrangement of light and dark areas that result from internal and external reflections.
- Contrast The relief of light and dark areas that creates the face-up pattern of a diamond.
- Weight Ratio A comparison of a diamond's weight in relation to its diameter.
- Durability This covers thin girdles as well as inclusions that weaken a gem.
- Polish The criteria remains the same, considering the overall quality of the polish, scratches, nicks, chips, etc.
- Symmetry Refers to the outline of the stone, facet arrangement and pointing, table and culet centering.
Also.... a few things to note.
Though inclusions can affect brilliance, they are not considered in the cut grade.
Cut grades only apply to round brilliant cut diamonds in the D to Z color range. Champagne colored diamonds cannot be fairly compared with near colorless stones.
Understanding the GIA's Cut Grading System
Cut is graded from Excellent to Poor. Gone are the classes of the previous system. There is little change in the top grades, but in the mid-range a lot more flexibility has been added. The grades are now based on the optical performance of the diamond, rather than mathmatical formulas. As such, they should give a more realistic assessment of a diamond's appearance.
You can no longer look up the criteria for grade. That is because each grade represents a range of proportion sets and diamond appearances which need to be considered together. While potentially more complex, the GIA has made software available to do the calculations. (See the illustration on the right.)
Even though the new criteria are more complex, a few of the changes can be seen. Very thin girdles no longer qualify in the top grade. Culet sizes are now more limited as well.
Twelve criteria need to be determined to complete a cut grade. Only a couple are new, Star and Lower Half percentages, but several of the others need to be measured to closer tolerances than before. For details on how to grade a gem with the new method, IGS members can go to Cut Grading Diamonds.
Changes to GIA Diamond Grading Reports (as of January, 2006)
Beginning on January 1, 2006, the GIA Laboratory began issuing updated versions of the GIA Diamond Grading Report and GIA Diamond Dossier®. These new reports will list diamond cut as Excellent to Poor. They will also contain expanded proportion data and a new diagram.
What to do if you have an old GIA Diamond Grading Report and need an update?
The information needed to create new reports has been collected by GIA since January 1, 2005. Most diamonds graded after this date will not need to be sent it for re-examination. Note that some diamonds may need to be returned to obtain updated measurement information.
In addition, a service to update the grading information to include a cut grade for reports will be provided for those stone graded before January 1, 2005.