By Bryan Boyne (g.g.) , Tuesday, December 29, 2015
At a time before laboratory cut grading, the American round brilliant came to be characterized by a larger table size than was the traditional style in Europe. Eventually a diamond featuring a 60% table and 60% depth (so called 60/60 diamonds) became a commonly referenced standard for a well cut diamond in the dominant US market, despite the fact that Marcel Tolkowsky had demonstrated mathematically the benefits of smaller tables all the way back in 1919.
Tolkowsky Ideal Cut
Manufacturers promoted a bigger table because they could get greater yield from the rough by cutting diamonds with shallower crowns (see ‘Effects of a Large Table Size’ below). Many people looked no further than depth and table percentages to determine whether a diamond was well cut, as there was nothing on the lab report to overall cut quality. Although tables even bigger than 60 were being cut, the 60/60 came to represent a high quality cut, despite the fact that such a combination could encompass many different overall proportion sets and critical crown and pavilion angles.
Extreme examples of poorly proportioned 60/60 diamonds
Calculating Depth and Table Percentages
DEPTH PERCENTAGE = The total depth of the diamond divided by the average diameter.
TABLE PERCENTAGE = The average width of the table divided by the average diameter.
While it is true, as illustrated in the profile views of the computer renderings above, that many unfavorable proportion combinations can result in 60/60 percentages, there are also combinations that have the potential to produce outstanding performance and which can earn GIA Excellent and even AGS Ideal grades. In both cases it is necessary to know much more about the diamond than just table and depth percentages.
Illustration courtesy of American Gem Society Laboratories (AGSL)
Effects of a Large Table Size
As illustrated below, cutting a larger table has the effect of making the crown shallower.
The shallower crown has the same number of facets, all of which become smaller. This limits both the size of the “virtual” facets, the ability of the diamond to gather light from lower angles, and it reduces the surface area on the crown capable of exhibiting fire (see “dispersed light” in ray tracing illustration below).
Illustration courtesy of University of Wisconsin-Madison
The following illustration shows that two diamonds can be cut, one larger and one smaller, from the most common crystal form of rough diamond - the octahedron. The shallower that the crown of the larger stone is cut, the more material is available to cut the second diamond, thereby increasing overall yield.
Well Cut 60/60 Diamonds
As mentioned earlier, it is possible for a diamond to be very well cut with a 60/60 profile. This combination can be rewarded the highest grade from GIA (EX) and even from AGS (Ideal). But 60/60 does not guarantee good cut quality- nor does it rule it out. Actual performance depends on how well the various angles and proportions work together. Below are charts for GIA and AGS showing the range of possibilities for a diamond with 60% table to earn the cut grades of Excellent and Ideal, the top grades from the respective labs.
GIA proportion chart for 60% table. Excellent candidates are in dark blue.
(click to enlarge)
AGS proportion chart for 60% table shows candidates for Ideal in red.
(click to enlarge)
Not surprisingly, there are far fewer candidates for Ideal in the more stringent AGS system. It should also be noted that the AGS light performance cut grading system involves an analysis of a 3D model of the diamond itself in which sophisticated ray tracing software evaluates the contribution of every facet in order to calculate a cut grade. The GIA system matches averaged and rounded measurements to predefined tables in order to designate a grade.
60/60 Diamonds - Pros and Cons
A cutter can improve yield by cutting a primary stone with a flat crown, thereby allowing for a bigger secondary stone. This tends to benefit the manufacturer; not necessarily the consumer.
Bigger tables with shallower crowns, if appropriately proportioned and cut well, can deliver outstanding white light return, but at the expense of some fire. As the top gets flatter the diamond performs more like a mirror and less like a prism.
A 60 percent table with a shallow crown and less depth than is typically required by a bigger crown can potentially be “spready”. That is, the weight is spread out over a larger diameter allowing the diamond to be slightly bigger in terms of outer dimensions.
What is sacrificed?
- As the table gets bigger, the surface area of the crown is diminished and crown facets become smaller. Since crown facets are instrumental in producing fire (colored sparkles), a 60/60 will usually not display as much fire as a well cut diamond with a fuller crown.
- Diamonds with lower crown heights are not able to tolerate tilt angles as well. The unattractive “fish eye “effect where the girdle reflection is visible at a small degree of tilt is more of a concern in diamonds with larger tables/lower crowns.
- The larger table facet will also throw bigger glare from a wider range of viewing angles. Glare will tend to conceal some of the positive aspects of light performance such as fire and scintillation.
- To some extent, as the table size increases inclusions become easier to see as the camouflaging crown facets are reduced. Think of an emerald cut with its traditionally large table facet and how revealing it is in terms of clarity features.
When GIA introduced their cut grading system in 2006 the market began to change rapidly due to their worldwide reach. Manufacturers were able to take advantage of the steeper and deeper crown and pavilion angle combinations that the GIA system allows, thereby getting good weight retention without the need to spread the table as much.
In general, cutting has improved dramatically with respect to the round brilliant as a result of the introduction of laboratory cut grades. Information technology has also been a driver toward quality as more consumers are learning the impacts of cutting on diamond light performance and beauty. The ability of jewelers and manufacturers to provide light performance images demonstrating the level of performance and faceting precision of their goods has increased consumer confidence, especially for those shopping online. And there is more competition than ever in the ideal and super ideal diamond market which has increased awareness, interest and demand for top cut quality in the consumer market.
There have always been those in the diamond trade who have advocated for cutting for beauty over weight. It is clear that the trend we see today is finally validating their philosophy. To paraphrase the great American diamond pioneer Henry Dutton Morse, who was one such advocate in the late 19th century,
“Judging diamonds by the carat is like judging a racehorse by the pound!”
*Though Marcel Tolkowsky gets most of the credit for defining the Ideal Cut in his 1919 publication, Morse and a few others were already cutting diamonds in America to similar proportions, and strongly advocating cutting for beauty over carat weight.
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