GIA Cut Grading Study

GIA’s New Cut Grading System for Round Diamonds

 Before we analyze GIA’s new cut grading system, it will be helpful to first give you some background information on the Gemological Institute of America and the American Gem Society.  Both institutions run laboratories and do fine research, but they are in fierce competition with each other to have the consumer use their certificates, as these certificates are a major source of their revenues.  Although each lab uses different scales for the color and clarity of a diamond, these scales can be referenced to each other.  It is on the subject of cutting that the two labs have differed widely. 

While both labs had been studying the subject of cutting, it was the AGS that first marketed their certificates with a cut grade in 1996, beating GIA by ten years.  They based their cut grade on the Tolkowsky mathematical model of a diamond designed in 1919 and used this as the basis of their “ideal” or “zero” cut. The AGS cut grading system employs eleven gradations labeled zero through ten, with zero being the best.  The table size of the “zero cut” was limited to a measurement of 53 to 57%.  GIA neither agreed with this approach nor conceded that this type of diamond cut yielded the most brilliant diamond.  In fact, AGS knew there were shortcomings to this system and put a warning to consumers on their certificates stating that diamonds of the same color, clarity, and cut grade could vary by as much as fifty per cent in price, and that a jeweler should be consulted for guidance.  Over the years I have seen many AGS “zero” cuts that I felt were lacking in beauty and brilliance, and often showed heavy extinction areas.  AGS has now refined their original cut grading system, currently using 3D modeling with proprietary software.  It appears that any number of diamonds formerly rated as “ideal zero” cuts may drop by as much as three grades in the new system. I suspect that many consumers who bought their diamonds based only on the certificate will by very unhappy.   

GIA, meanwhile, took the approach that there was more than one way to cut a nice diamond. Their first cut grading system consisted of four categories designated 1, 2, 3, and 4, with parameter ranges stipulated for each. GIA often stated that there was no one set of parameters that always worked with every stone, and that the final determination of the cut quality should be determined by the gemologist.  Most distinctive between the two cut grading systems was always the element of table size.  For GIA’s class 1 cut grade, the table size could range from 53 to 60%.  The class 2 cut grade allowed a table size of 61 to 64%, also a dimension that could enable a diamond to potentially have optimal beauty.  Generally speaking, diamonds in cut class one and two had dimensions that could potentially produce beautiful diamonds, while those in cut grades 3 and 4 did not. This grading system was to be used as a guideline for gemologists and was not promoted on their certificates. (Some research gemologists thought that having only four grades was too broad, and subdivided each category into two parts for a total of eight grades. This, however, was never sanctioned by GIA).  

Meanwhile, GIA stepped up their research, but it was not until January, 2006, after many delays, that the new cut grading system for round diamonds was formally initiated.  The new system measures seven different parameters of a round diameter before computing a cut grade for the stone.  These grades are: excellent, very good, good, fair, and poor. It should be noted that, in the new system, the maximum table size now allowed for a diamond in the “excellent” class was raised to 62%.  As an added note, AGS’s new cut grade system is abandoning the 53-57% table concept, allowing diamonds with up to a 61% table to be a candidate for their top, or “zero,” cut. 


While I have to commend GIA for their fine research in this area, I feel that their new system has many of the same consumer pitfalls as the AGS system.  First of all, in my opinion, their categories are once again too broad enabling too many diamonds of marginal qualities to attain a cut grade of “excellent” and “very good”. (GIA has tried to put all the various cut combinations into just five categories). There are many in the industry that believe that GIA widened parameters as a result of complaints by various cutters that the system was not viable from a  practical cutting standpoint (perhaps this was one of the reasons for the many delays).   During their research, GIA disclosed very little to the general trade, but did solicit feedback from select clientele. Their official stance was that their system would allow for a “wide variety of cutting styles and tastes.”  Their “very good” cut grade includes diamonds that would previously have been in GIA’s former cut class 3, a grade many gemologists find totally unacceptable. Their “very good” and “good” cut grades seem to be mostly  “jeweler friendly” grades. 

I find that many of the diamonds in GIA’s top cut class of “excellent”, while they possess the proper measurements, do not have optimal light performance. In fact, they tend to exhibit many of the characteristics of the diamonds that AGS previously graded as “zero,” but may now be downgraded in their new system.

In summary, the consumer must realize that GIA’s new system is a system of measurements, and not necessarily of light performance, these two characteristics often being mutually exclusive.  For the consumer’s benefit and for appraisal purposes, I find it necessary to distinguish diamonds that fall into the top cut grade solely on the basis of their measurements from those that not only have the proper proportions but also exhibit optimal light performance resulting in exceptional beauty.  M. Martin and Co. will distinguish these diamonds by adding our “Best of Class Diamond®” designation. Therefore, a diamond we judge to be of exceptional beauty in GIA’s top cut grade will be distinguished as “excellent cut, Best of Class Diamond®”.
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Below are the most frequently asked questions I have been receiving: 

How do you sum up GIA’s new cut grading system? 

While I feel that GIA has made a positive step in the right direction, they still need to refine the system. I expect that we will see changes in the system in the coming years. For now, the advantage for the consumer is that it will prevent the jeweler from selling them substandard cuts as good stones.  The disadvantage will be to the consumer who becomes totally reliant on what the certificate states and is misled to believe that all the diamonds they grade as “excellent” will be beautiful stones.  The cut grade is assigned to the diamond essentially on the basis of its measurements as determined by a computer program, and not by any visual observations by the graders.   

Can two diamonds of the same size, color, clarity, and cut grade be priced differently? 

Absolutely. For example, diamonds in the lower parts of any cut grade (that is, diamonds have the measurements that place them in a certain class but are not beautiful diamonds) will be discounted in the marketplace. A diamond that receives an “excellent” grade by its measurements but is not optically beautiful will be purchased by the jeweler for substantially less than a beautiful diamond which rates high in the same cut class. The consumer may then think that he is purchasing a high quality diamond at a bargain price. 

Why have the labs been working on cut grading for so long?

Cutting is an extremely complex subject.  First of all, the consumer must realize that all the research GIA has done so far has only been on the round diamond, the shape they understand most.  As stated by William Boyajian, then president of GIA: “ The [cut grading] project to find these parameters involves creating computer models of diamonds of various shapes, proportions, and facet arrangements, and then testing how light interacts with the diamond considering factors such as the light source, the optical properties of the diamond, the geometric relationships between the light source, the diamond and the human eye; and how the human eye works.”  GIA researchers concluded from their findings that “the relationship between brilliance and the three primary proportion parameters (crown angle, pavilion angle, and table size) is complex.  The way these three factors interact with each other can affect brilliance in sometimes unexpected ways.”  Perhaps the consumer should liken this to having a computer scan a Picasso painting, digitize it, and then having the computer program try to determine how beautiful it is. 

Will this prevent cutters from shifting crown and pavilion angles to cut “paper pretty” stones?

No.  First of all, let us discuss that this means.  “Crown-pavilion angle shifting” is a technique used by the cutter to leave excess weight on the diamond without it showing up on the certificate.  Here is how it works:  Two of the significant figures on a certificate are the depth percentage and table percentage of the diamond.  The depth percentage of a diamond is calculated by measuring the total depth and dividing it by the width of the diamond.  Holding the table size within proper parameters, the cutter can cut the crown of the diamond higher than it should be while reducing the depth of the pavilion by a similar amount (he can also do the opposite—cut the pavilion deeper than it should be while reducing the crown height a similar amount).  By doing this, he keeps the total depth of the diamond the same.  So even though the crown and pavilion angles would be wrong, the depth percentage and the table percentage would look correct on the certificate (thus the term “paper pretty”).  Even with the new cut grading system, this technique can still be used on round diamonds, though it will be slightly more constrained.  It can be, and is, commonly used on all the other shapes. 

How do members of the GIA lab view the new system?

To answer this, let us look as some quotes from directors at GIA’s laboratory:

Quote from Barak Green, Gem Trade Labs’ communication manager: “When people looked at diamonds in the observation tests, there was no stone that everyone agreed was the best diamond.  People could not agree on the best set of proportions.  As a result, the stones that receive the top grade of “excellent” can look quite different.” 

Quote from GIA researcher Al Gilbertson: “If consumers rely solely on the paper (certificate), they will see a wide variety of appearances, and they will be puzzled at that. So what a concept–people will need to look at the diamond they are going to wear for the rest of their life.” 

How does all this play out in the marketplace? 

I am seeing a big demand by dealers for the lowest ranking diamonds in the top three cut classes.  A lot of this demand is also coming from internet companies who are looking for the lowest price diamonds in each cut class.  We, too, will be selling these types of diamonds to our dealers, although we certainly don’t recommend these diamonds to our private customers.  However, we will always be glad to show you these diamonds next to our Best of Class Diamonds®.  The reason we don’t recommend these diamonds is twofold: first, of course, is that they do not possess optimum beauty; and second, we anticipate that they will be downgraded in the future as GIA refines their system.  

Comparison of Cut Grading Systems

Previous AGS System

New AGS System

New GIA System

Number of Cut Categories




Categories based solely on the
Tolkowsky mathematical model




Certificate contains warnings or disclaimers regarding limitations of their system.

Yes. Certificate contains warning to consumers that diamonds in the same cut grade can look different and vary by 50% or more in price.

Yes. Certificate contains warning to consumers that diamonds in the same cut grade can look different and vary by 50% or more in price.

Yes. Warning on front to consult a gemologist. Has numerous disclaimers on back.
Number of visual and physical characteristics used to determine cut grade





“Best of Class Diamonds®” is a registered trademark of M. Martin and Company.  For a round diamond to receive this designation, it must rank in GIA’s top cut grade of excellent, this grade being assigned to the diamond by GIA on the sole basis of its physical measurements. With respect to all other diamond shapes, for which no GIA cut grading system currently exists, the diamond must be of fine cut to accepted modern gemological proportions.   To attain the “Best of Class Diamonds®” designation, the diamond must possess a proper balance of brilliance, dispersion, scintillation, and other optical characteristics, and it must exhibit superior overall light performance.  In addition, the diamond is required to have the laboratory grade of “very good” or “excellent” for both polish and symmetry.  Not all round diamonds with GIA’s cut grade of excellent qualify to receive our “Best of Class®” designation.
In summary, when it comes to determining the light performance characteristics and the diamond’s resulting beauty, neither of the lab reports offer enough information to buy a diamond based only on the certificate.

Copyright M. Martin and Company, 2007, 2013, 2015.

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