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Cut: The 4th C
Research and Discovery
The Questions behind Cut
The Human Factor
Observation Results
Putting It All Together


 

Cut: The 4th C

Of the 4Cs, Cut has remained the least understood - and the least agreed upon. Many in the trade have been strongly in favor of a cut grade; while others have questioned whether an objective, comprehensive system could even be developed. And if it could be developed, would it be useful?

In an effort to establish standards for cut in the same manner as it has for color and clarity, GIA has invested in the research of diamond cut for more than 15 years. This exploration supports GIA's mission to ensure the public trust in gems and jewelry by upholding the highest standards of integrity, academics, science, and professionalism, through education, research, laboratory services, and instrument development. (back to top)



 

Research and Discovery

GIA had been studying diamond cut for decades, when in 1989 the Institute turned a significant corner. It was at this time that new advances in computer technology allowed GIA researchers to analyze aspects of the appearance of round brilliant cut diamonds that once had been impractical and difficult to explore. 

Using advanced computer modeling, we were able to replicate the way light behaves within a round brilliant cut diamond so that we could predict how a diamond would perform with regard to brightness and fire. To study these two aspects of face-up appearance, we analyzed tens of thousands of proportion sets. These proportion sets included: table size, crown angle, pavilion angle, star length, lower-half length, girdle thickness, culet size, and total depth. We also studied the effects of polish and symmetry. We found that all of these parameters were interrelated in their effects on light performance, and that no single proportion, or subset of proportions, could be considered alone. In addition, we found that there was a wide range of proportions that had the potential for high quality light performance.

But that was just the beginning. (back to top)



 

The Questions behind Cut

We wanted to answer two important questions: First, to what degree do changes in proportion values create observable distinctions in appearance? And second, which proportion sets produce results that are deemed attractive or unattractive by most observers? We knew the answers to these questions would be critical in creating a system of diamond cut assessment. This system had to be scientifically accurate, as well as practical and useful for the diamond industry and the public.

What began as a project to develop a highly theoretical computer model of a "virtual" diamond using sophisticated ray-tracing software soon evolved into a multi-million dollar study that encompassed every aspect of cut appearance and quality in round brilliant diamonds. (back to top)



 

The Human Factor

The GIA Diamond Cut Grading System is a predictive system that is grounded in science and research. But we realized for it to be truly practical and applicable in the jewelry industry, this system needed to be tested by members of the trade and the public in a standardized viewing environment. Did scientific discovery translate to practical application? It did.

To refine and validate our computer metrics for brightness and fire, as well as analyze other components that contribute to a round brilliant's overall cut appearance and quality (such as scintillation, design, and craftsmanship), we embarked on an extensive journey of observation testing and trade interaction.

Over 70,000 observations were made on more than 2,300 diamonds, using observers from all sectors of the jewelry industry: diamond manufacturers, dealers, retailers, and potential customers. This undertaking was, without a doubt, the largest study of this type ever conducted. To our knowledge, no other organization or research group has validated their models with observation testing of actual diamonds to the same extent GIA has. (back to top)



 

Observation Results: What You See Is What You Get

Each observer was asked to evaluate the brightness, fire, and overall cut appearance of diamonds representing many different proportion combinations. The results of this observation testing were vital to the development of the GIA Diamond Cut Grading System and can be summarized as follows:

1. We were able to validate our computer modeling for brightness and fire. This included refining our metrics so that the results represented the brightness and fire seen in actual diamonds when they were observed in typical lighting and viewing conditions.

2. We were able to confirm, analyze, and eventually predict other components of face-up appearance, such as scintillation, as well as essential additional components of cut quality such as design and craftsmanship.

3. We were able to determine how much of a difference in any one component (for example, levels of brightness) observers could see consistently in actual diamonds. Similarly, it allowed us to determine how many overall cut grade categories people could consistently discern. The value of these results was immeasurable — they allowed us to transform our scientific models into a practical system that could be used in the day-to-day world of the diamond industry.

4. We discovered that personal and regional preferences were an inherent part of a truly functional cut grading system. Although we confirmed that most observers could discern five overall grade categories for diamond cut quality, personal and/or regional tastes often dictated which diamond was finally preferred within each of those categories. This underscores the fact that there can be many different, yet equally pleasing, appearances within a grade category.

5. Perhaps most importantly, extensive observation testing and trade interaction allowed us to create a diamond cut grading system that was accurately predictive. That is, it assured us that when the complete set of proportion and parameter information is input into the system to obtain a cut grade, that grade is consistent with the grade that would have been provided by a large majority of observers. It is this predictive nature of the cut system that allowed GIA to create a cut database that contains cut grading results for over 38.5 million proportion combinations! (back to top)



 

Putting It All Together

Through computer modeling, observation testing, and trade interaction, we confirmed that to be attractive, a diamond should be bright, fiery, sparkling, and have a pleasing overall appearance, especially when the pattern of bright and dark areas is viewed face up.

In the same manner, we recognized that more than just face-up attractiveness should be incorporated into the assessment of overall cut quality. Quality in design and craftsmanship (as evidenced by a diamond's weight ratio, durability, polish, and symmetry), even when face-up appearance is barely affected, also should be considered in a diamond's final cut grade. (back to top)