Ideal Cut Diamond Proportions

This article has two sections. The first section will provide our best-summarized insights on ideal proportions for  round and non-round fancy-shaped diamonds. Please note that the GIA has yet to assign a cut grade to fancy cut stones, such as  princess cut or cushion cut, but these proportions are still critical in analyzing fancy-shaped diamonds' cut. 

Summary of Ideal Proportions in a Diamond

Round Cut Diamond:

Compare These Proportions with JamesAllen.com 3D Video Technology.

Please note that crown angle and pavilion depth play a significant role in picking a super ideal cut diamond from an ideal cut diamond. This information proves helpful while reviewing two round diamonds with an equally excellent cut, symmetry, and polish grade from the GIA. Of course, these proportions also apply to  lab-created diamonds. 

Check An Example of AGS Super Ideal Cut Diamond Here. 

Important Clarification:

Click here to learn more about the  importance of cut among the 4Cs. But while the cut is essential, the brilliance of a diamond is about all 4Cs. Therefore, it would be an ideal scenario if you can obtain the above mentioned super ideal proportions without  compromising on the other 3Cs, However, do not compromise overly on the other 3Cs to get the best proportions; your approach needs to be balanced. Moreover, it is okay for the proportions to be slightly high or low. The GIA states that you could model these proportions slightly differently to get the same level of brilliance ( a tolerance of 0.2-0.3% up or down is perfectly acceptable).

Fancy Cut Diamonds:

Follow the chart below for fancy-cut stones. You may read our detailed guide for each shape before making a purchase.

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, the cut is critical in determining brilliance. Please note that basic knowledge about diamonds is required to understand the substance of this article. But if you are new to this field, we recommend using our simple  guide instead.

The 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 an optimal level of brilliance. Put differently; it could be argued that there are many ways to proportion a diamond for maximum brilliance. These findings confirm the commonly held view among cutters that a diamond does not need to have a prescribed depth or table (among other factors) to look as brilliant as one that meets rigid parameters. Proportion can be modeled differently to achieve optimal brilliance.

The key takeaway from these findings is that while these parameters are essential, they can only take us so far. We should not 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 with ideal proportions. We also think that the issues of fire and scintillation should also be considered while evaluating the overall brilliance of diamonds.

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

GIA's Look into Brilliance:

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

The term "brilliance" refers to the brightness of a diamond and is determined by the amount of light reflected from within. Here is how it works: light enters the diamond through the "table" or the top of the diamond and moves through a spectrum of colors as it bounces off the mirror-like facets and runs across the interior. When the light leaves the diamond, it is reflected in 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 by cutting facets, typically performed with a faceting machine. The ideal facet cut will produce the highest amount of brilliance possible from the stone. If an expert artisan cuts the facets, there will be no loss of light when it is reflected in a diamond. The less light reflected from 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 of brilliance.

How is Brilliance Determined?

The brilliance of a diamond is measured using the (WLR) Weighted Light Return. A WLR is a numerical evaluation obtained by tracing and weighting rays of light as they pass through the interior facets of a diamond. No two diamonds will be identical. The GIA's standard lighting state simulates the average amount of daylight when diamonds are viewed. The analysis also considers how diamonds are moved when they are being looked at as well as glare, which could be incorrect as WLR.

The researchers at the GIA had determined that the relationship between brilliance and the three primary proportion parameters (pavilion angle, crown angle, and table size) is complicated.Ultimately, the combination of the factors and how they interact determines the brilliance - any change will affect it unpredictably.

At about the same time that the GIA was looking into the brilliance of diamonds, a group of scientists from the Moscow State University produced a study on diamond brilliance. The study, led by Yurri Shelemetiev and Sergey Sivovolenko, included a theoretic result involving light return and fire. Using a virtual model analysis, unlike the GIA, they used a realistic lighting environment. The researchers emphasized that light return should not be equated with 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 individual's eyesight and the angle at which they are looking at the stone, their opinion about the brilliance of the diamond might change or defer.

GIA's Modeling by Computer:

In the 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 wholly symmetrical and had perfect polish. Once that had been established, they added eight-cut factors and some physical ones 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, the GIA determined the individual factors of a diamond that result in its brilliance. Any diamond cut and physical characteristics can dramatically alter how much brilliance a diamond has.

As time passed, the demand that the GIA provides cut grades rose. However, it refused, knowing that since so many variables go into the overall appearance of a diamond, one cut will not be better than another. Instead, it created 20,122 proportion combinations in its simulation, mixing crown angle, pavilion angle, and table size while keeping the extra five factors constant. In the simulation, 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, the 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.

A 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 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.

Around the 13th century in Europe, diamonds became associated with the monarchies and aristocracies, set against pearls and gold as accent pieces. As centuries went on, diamonds became more prominent features in jewelry for the wealthy, which 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 were greatly enhanced, making them more attractive to wear than ever before. In the 17th century, the diamond became the small staple gemstone in high-class jewelry. By the 18th 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 world's royalty tried to obtain diamonds throughout the centuries, it was not until the end of the 19th century that the role of diamonds faced a change. The first event that affected them was the discovery of immense diamond deposits in South Africa in the 1870s. This meant that diamonds could be procured much more efficiently than ever before, and you did not need to be royal to have one. The other big event was that, 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 to a level that had never been seen before.

As a result of Tiffany & Co.'s discovery and presentation of the crown jewels, jewelers learned the importance of displaying diamonds with the ideal illumination to make the stones' brilliance stand out. Since where the light is coming from will affect the look of a diamond's brilliance, fire, and sparkle, most jewelers display diamonds in bright spotlights. If the jeweler wants to present the diamond's brilliance to the highest degree, they may use fluorescent illumination. However, doing so may come 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