This article has two sections. In the first section, we will provide our best summarized insights on ideal proportions for round and non-round fancy shape diamonds. Please note that GIA does not assign a cut grade to fancy cut stones such as princess cut or cushion cut yet, but still these proportions are critically important in analyzing the cut of fancy shape diamonds.

Summary of Ideal Proportions in a Diamond

Round Cut Diamond:

Click hear to learn more about the importance of cut among the 4Cs. While cut is absolutely critical, the brilliance of a diamond is about all 4Cs, not just cut. If you can obtain the above super ideal proportions without compromising on the other 3Cs, it would be an ideal scenario. However, do not compromise overly on the other 3Cs in order to get the best proportions. It needs to be a balanced approach. Moreover, it is okay for the proportions to be slightly high or low – GIA concluded that you can model these proportions slightly differently in order get the same level of brilliance (a tolerance of 0.2-0.3% up or down is perfectly acceptable).

The issue of Diamond Brilliance:

Brilliance is hailed as the ultimate factor in determining the overall beauty of a diamond. As you might have heard by now, when it comes to diamonds, the cut of a diamond is critical in determining the overall brilliance of a diamond. Please note that basic knowledge about diamonds is required to understand the substance of this article.

GIA refers to brilliance as “the intensity of the internal and external reflections of white light to the eye from a diamond or other gem in the face-up position” and measures it in terms of Weight Light Return (WLR).

After many studies and models, the industry has come to some consensus around the proportions of a diamond that will establish the quality of its cut.  Cut plays an important role in determining the brilliance of a diamond. As an example, it might be helpful to check these parameters for a round or cushion shape stone and see how they play a role in determining their cut.

The findings of the GIA study are rather interesting. It indicates that while proportions such as the ones developed by Mercel Tolkowsky for an ideal cut diamond are important, it is not critical for a diamond to be within these rigid parameters to obtain optimal level of brilliance and/or that there many ways to proportion a diamond for maximum brilliance. These findings confirms the commonly held view among cutters that a diamond does not have to have a prescribed depth or table (among other factors) to look as brilliant as one that meets rigid parameters or that proportion can be modeled differently to achieve optimal brilliance.

The key take away from these findings is that while these parameters are important, they can only take us so far and that we shouldn’t mark diamonds that are not precisely within the proportions of one grading model as inferior without inspecting their visual appearance. In some cases, their visual appearance might even surpass the brilliance of ones that have ideal proportions. We also think that the issue of fire and scintillation should also be taken into consideration while evaluating the overall brilliance of diamonds.

Our advice to customer would be to not spend too much time on the issue of brilliance as it is an advanced (confusing) topic for regular consumers to grasp easily. Instead, we would recommend going with either GIA or AGS grading systems and trusting their assessment. Since GIA does not offer a cut grade for shapes other than round, follow the different general parameters for each shape diamond in order to get a well cut stone.

GIA’s Look into Brilliance:

In the late 1990s, the Gemological Institute of America or GIA conducted a study of brilliance. The most important insight from the study was that the proportions of a diamond can be modeled differently to achieve the same level of optimal brilliance.

The term “brilliance” refers to the brightness of a diamond. This is determined by the amount of light that is reflected from within the diamond. How it works is that light enters the diamond through the “table” or top of the diamond. The light is then moved through a spectrum of colors and reflected all throughout the interior of the diamond, bouncing off of the mirror-like facets before the light exits and is reflected back to the eye.

The GIA’s definition of brilliance in their Diamond Dictionary is “the intensity of the internal and external reflections of white light to the eye from a diamond or other gem in the face-up position.” Brilliance is ultimately created through the cutting of facets, typically performed with a faceting machine. The ideal facet cut will produce the highest amount of brilliance possible from the stone. If the facets are cut by an expert craftsman, there will be no loss of light when light is reflected in a diamond. The less light that is reflected back to the eye, the lower the amount of brilliance a stone has. Light is lost when the angle of the facets shoots the light upward or downward, both of which result in a lower amount brilliance.

How Is Brilliance Determined?

The brilliance of a diamond is measured using the (WLR) Weighted Light Return. WLR is a numerical evaluation that is obtained by tracing plus weighting rays of light as they pass through all of the interior facets of a diamond. No two diamonds will be identical. GIA’s standard lighting state was able to simulate the average amount of daylight when diamonds are viewed. The analysis also took into account how diamonds are moved when they are being looked at as well as glare, which could be incorrect as WLR.

The researchers at GIA had determined that “The relationship among brilliance as well as the 3 primary proportion parameters (pavilion angle, crown angle plus table size) is difficult…” Ultimately, it is the combination of the factors and how they interact with one another that determine the brilliance since any altered factor will affect it in unpredictable ways.

About the same time that GIA was looking into the brilliance of diamonds, a group of scientists from Moscow State University produced a study about diamond brilliance. The study led by Yurri Shelemetiev and Sergey Sivovolenko, included a theoretic result involving light return as well as fire. Using a virtual model analysis, unlike GIA, they used a realistic lighting environment. The researchers emphasized that the light return should not be equated with the brilliance and that human physiology was also a factor in determining an individual stone’s brilliance. The main reason behind this finding was that depending on the eye sight of the individual and the angle the individual is looking at the stone, their opinion about the brilliance of the diamond might change or defer.

GIA’s Modeling by Computer:

In GIA’s study on measuring the brilliance of diamonds, they utilized a computer that simulated a three-dimensional model of a round brilliant, flawless, colorless fifty eight-facet diamond that was completely symmetrical and had perfect polish. Once that had been established, they then added in eight cut factors as well as physical factors that affect how light interacts with a diamond. The cut factors were:

Table size

Lower girdle length

Cutlet size

Star length

Girdle thickness

Girdle facets

Crown angle

Pavilion angle

Through the computer simulation, GIA was able to determine all of the individual factors of a diamond that result in its brilliance. Any of the cut and physical factors can dramatically alter how much brilliance a diamond has.

As time has gone on, there has been a high demand for GIA to provide cut grades, but GIA refused on the grounds that since so many variables go into the overall appearance of a diamond, one cut is not going to necessarily be better than another. In their simulation, GIA created 20,122 proportion combinations, mixing crown angle, pavilion angle, and table size while keeping the extra 5 factors constant. In the simulation, the brilliance was determined using the WLR.

Probably the most astounding discovery from the simulation was that some of the most popular diamond cuts, including the American Ideal, produced the least amount of brilliance from their diamonds. But even with that result, GIA said that the findings should not be used as a brilliance grade for all diamonds since it is not just the cut that affects the brilliance of a diamond.

History of Diamond Proportions and Brilliance:

Diamonds have been used for various purposes throughout history. The first mention of diamonds in ancient texts was from a Sanskrit manuscript written sometime between 320 and 296 by a minister in northern India. But while diamonds have been used for millennia, the history of the brilliance of diamonds is not quite as long.

It was around the 13^th century in Europe that diamonds became associated with the monarchies and aristocracies, set against pearls and gold as accent pieces. As the centuries went on, diamonds became bigger features in jewelry for the wealthy, and this was greatly due to the understanding of diamond faceting. When diamond faceting was developed, the jewelers of the time found that the brilliance and fire of the diamonds was greatly enhanced, making them more attractive to wear than ever before. In the 17 ^th century, the diamond became the staple small gemstone in high-class jewelry. By the 18 ^th century, they had also taken over as the most sought-after large gemstones. This increase in interest would not have happened to the same degree if the brilliance of diamonds had not been studied. Since the more brilliance a diamond has, the more it reflects light, royals wearing diamond pieces would sparkle, making them seem even more impressive than they would have been otherwise.

While the royalty of the world tried to obtain diamonds through the centuries, it wasn’t until the end of the 19 ^th century that the role of diamonds faced a change in themselves. The first event that affected diamonds was the discovery of immense diamond deposits in South Africa in the 1870s. This meant that diamonds could be procured much easier than ever before and you didn’t need to be royal to have one. The other big event was, following the fall of Napoleon III of France, the French crown jewels were sold to Tiffany & Co. of New York and taken to the United States. Under the electric and gas lighting that had been steadily increasing around New York at the time, the brilliance of the diamonds was able to show off on a level that had never been seen before.

As a result of Tiffany & Co’s discovery and presentation of the crowned jewels, jewelers learned the importance of displaying diamonds with the ideal illumination to make the brilliance of the stones really stand out. Since where the light is coming from will affect the look of a diamond’s brilliance as well as its fire and sparkle, most jewelers display diamonds in bright spotlights. If the jeweler’s goal is present the diamond’s brilliance to the highest degree, they may use fluorescent illumination, but at the cost of dulling the fire and sparkle of the diamond.

 

Cut	Depth%	Table%	Crown Angle	Pavilion Angle	Girdle	Culet
Super Ideal Round Cut	60-62.1%	54-58%	33.9-34.9	40.2-41	Thin – Slightly Thick (2-3.5%)	None
Ideal Round Cut	59-62.8%	53-59%	33.9-35.5	40-42	Thin – Slightly Thick (1.5-4%)	None
Polish – VG to Excellent for Ideal and Excellent for Super Ideal
Symmetry – VG to Excellent for Ideal and Excellent for Super Ideal

Most Fancy Cut Diamonds:
Cut	Depth%	Table%	Girdle	Culet	Length to Width Ratio
Ideal Princess Cut	64-73	62-69	Thin-Slightly Thick	None to Very Small	Square: 1.00-1.05
					Rectangular: 1.49 -1.73
Ideal Oval Cut	57-62	53-64	Thin – slightly thick	None to Very Small
Ideal Radiant Cut	61-67	62-70	Thin – slightly thick	None to Very Small	Square: 1.00-1.05
					Rectangular: 1.20 -1.31
Ideal Cushion Cut	61-67	61-68	Thin – slightly thick	None to Very Small	Square: 1.00-1.05
					Rectangular: 1.2 -1.25
Ideal Emerald Cut	61 – 67	62-70	Thin – slightly thick	None to Very Small	Square: 1.00-1.05
					Rectangular: 1.39 -1.50
Ideal Asscher Cut	61-67	62-70	Thin – slightly thick	None to Very Small
Ideal Marquise Cut	57-62	53-64	Thin – slightly thick	None to Very Small