17.3 Cut grading

What Are Design and Craftsmanship, and How Do They Affect the Estimation of a Standard Round Brilliant Diamond's Cut Grade? 


In addition to a diamond’s optical attributes, the GIA Diamond Cut Grading System also takes into consideration its design and craftsmanship. This gives a well-crafted diamond the recognition it deserves while properly identifying the factors that make another diamond’s appearance less than pleasing. Both design and craftsmanship reflect decisions made during the fashioning process.

Design factors include the overall physical shape and proportions of a diamond, and how they affect weight and durability. The GIA Diamond Cut Grading System downgrades diamonds that carry excess weight in extremely thick girdles, steep crowns, or deep pavilions. Diamonds with features that make them vulnerable to damage – such as extremely thin “knife-edge” girdles – also tend to receive lower GIA cut grades.

Craftsmanship takes into account the care taken later in the finishing process. Ratings for polish and symmetry are included in this category.

The first step in any evaluation of design and craftsmanship is to measure the diamond’s average girdle diameter and consider its relationship to the diamond’s weight. Understanding this relationship can help you determine the maximum potential cut grade for a given diamond.


To determine diameter, measure from one edge of the diamond directly across to the opposite edge. Illustration by Peter Johnston.

Diamonds are measured in millimeters. For a round brilliant, the process begins with measuring the girdle diameter from one edge directly across to the opposite edge.

1. Measure the diameter in several places.

2. Add the smallest and largest diameters and divide by two to average the numbers.

3. Round your result to the nearest hundredth (0.01) of a millimeter.

Example: If minimum girdle diameter = 3.54 mm and maximum girdle diameter = 3.57 mm, then

Average girdle diameter = (3.54 + 3.57) ÷ 2

= 7.11 ÷ 2

= 3.555, round to 3.56 mm


What is the American Ideal Cut?


What exactly is the American Ideal? Was it Americans who refined the cutting/proportioning process?

The American Ideal Cut

Ideal is defined as:

"A standard of perfection, beauty, or excellence."

The standard of perfection and beauty in diamond cutting is embodied in the round brilliant, American Ideal cut. This concept of an ideal cut for diamonds was adopted, refined, and championed by both the GIA and the AGS for over half a century. Both organizations credited the Belgian diamond cutter and mathematician Marcel Tolkowsky (1899-1991) with the key design proportions of the American Ideal, (average pavilion and crown main angles of 40.75 and 34.5 degrees).

The GIA diamond course from 1993, on the page titled "The Quest for the Ideal" points out: "Although Diamond Design was first published in England, Tolkowsky's design is often called the American Ideal Cut, because US cutters were the first to adopt it. ... For years, GIA used the American Ideal Cut as the basis of a comparison system in teaching diamond cut evaluation."

Tolkowsky wrote of a breed of diamond cutter whose goal was to cut a diamond "regardless of loss of weight, the only aim being to obtain the liveliest fire and the greatest brilliancy."

Although not the first person to formulate proportions believed to result in maximal brilliance and fire, he is the one most people think of as the father of this ideal.

Tolkowsky's findings became the basis for the American Ideal Cut. And his model for the best-performing round-brilliant diamond remains the most influential in history.

Tolkowsky never called his diamond an "ideal cut." He called it a "high-class brilliant," meaning it had, in his words, "magnificent brilliancy and fire." What's more, he never said he invented his model for the round-brilliant diamond. His "Diamond Design" publication added mathematical support for the rightness of a model already in use. Its origin, he said, lies with others, including an almost forgotten mid-19th century Boston cutter named Henry Morse.

The beginnings of what came to be the American Ideal Cut diamond are traced to Henry Morse, who, in 1860, opened a diamond-cutting firm in Boston, Massachusetts. By 1880 he was recutting the traditional Old European cut diamond without regard for weight retention to produce the most beautiful round brilliant cut diamond of his time. (Morse's model had a pavilion angle within a degree of 41 and a crown angle close to 35 degrees).

His work went largely unheralded because the great majority of diamond manufacturing in the US steadfastly followed traditional European cutting with the goal of maximum weight retention from the rough diamond crystal.

Since the times of Morse and Tolkowsky, cutters of the American Ideal have continued to improve its beauty by lengthening the lower-girdle facets and slightly increasing the table size--all the while staying close to Tolkowsky's and Morse's recommended average pavilion and crown angles (40.75,41 and 34.5,35).

During the 80's, diamond cutters in Japan improved the physical and optical symmetry of the American Ideal cut using instruments that displayed the three-dimensional optical performance of all 57 facets of the round brilliant. The result was a new dimension in cutting perfection that today is called 'optical symmetry'.

Introduced in America in the 90's, the optically symmetrical, 'super ideal' was championed by several American diamond importers and cutting houses. The American Ideal continued to be refined by cutters using the latest diamond cutting technology and optical performance assessment tools.

The American Ideal cut diamond has evolved and improved in optical performance and beauty through the advancing skills and technology of the very best of the diamond cutting profession. Today, cutting firms worldwide are embracing the evolved American Ideal.

Begun in America with Henry Morse, championed by GIA and AGS, and nurtured by American cutters, this pinnacle of performance and beauty represents, in diamond cutting, the culmination of the quest for Ideal.

Photograph of an American Ideal, round brilliant cut diamond shown from a viewing angle of 40 degrees at six different magnifications, 2.5X-10X. This photo displays the American Ideal's potential for fire in high contrast illumination. Fire is the term for the reflections of light dispersed by the diamond into rainbow colors.


Fire is an important and desirable aspect of diamond beauty. It is a property of the American Ideal that often goes unheralded, because the bright lighting in today's jewelry stores usually emphasizes the other two aspects of diamond beauty--- brilliance and sparkle. 

The diamond's fire is better observed in high contrast lighting that is less intense. Lighting of this nature may be found in some restaurants, theatres, ballrooms, etc., where the illumination is mainly from spot lighting in otherwise low light surroundings. The prevalence of artificial lighting of this type from gas and kerosene lamps, candles, chandeliers, etc. better highlighted the fire aspect of the diamond's beauty in times prior to the advent of modern, 20th century lighting.

The Official Grades of Cut
Cut grade standards are defined by the GIA in the following terms:
Excellent (Xc), Very Good (Vg), Good (G), Medium (M), and Poor (P). The proportion is graded separately to the polish, proportion coming first. So for example an Xc Vg grade refers to a diamond that is Excellent Proportion with Very Good Polish.
The Effect of Cut on Price

A poorly cut diamond is typically 30% cheaper than an ideal cut diamond, where all other quality characteristics are the same.

GIA bases their new (as of Jan. 2006) cut grade on a combination of face-up appearance, design and craftsmanship elements that all contribute to the diamond's fire and brilliance. They employ a predictive computer model,       
GIA cut grading criteria
Face-Up Appearance     Design     Craftsmanship

Brightness                     Weight Ratio     Polish
Fire                                 Durability     Symmetry

based on over 70,000 individual diamond observations and 38.5 million proportion sets, to determine a diamond's brilliance based on its interrelated measurements. Most GIA diamonds graded prior to January 1st, 2006 will not have a laboratory-assigned cut grade. The GIA cut grade system includes ratings of Excellent, Very Good, Good, Fair and Poor. GIA does not assign an Ideal cut and an Excellent rating is their highest grade. For comparison purposes, a GIA Excellent cut will be listed as a Blue Nile Ideal cut on our Web site. The cut grades are equivalent. Learn more about the GIA Diamond Grading Report and Diamond Dossier®.

How Does Total Depth Percentage Affect the Estimation of a Diamond's Cut Grade? 


Total depth percentage is an important indicator of a stone's overall proportions. It can help you recognize an underweight or overweight diamond. Illustration by Peter Johnston.

The total depth percentage of a round brilliant diamond is its table-to-culet depth, expressed as a percentage of its average girdle diameter. Total depth percentage is an aspect of design, and therefore important to the assessment of an overall cut grade. The total depth percentage and the diamond’s potential cut grade can be determined by following the procedure below.

1. Calculate the diamond’s average girdle diameter.

2. Measure the diamond from table to culet.

3. Divide the depth in millimeters by the average girdle diameter, and multiply by 100.

4. Round the result to the nearest tenth of a percent.

5. Use the table below to help estimate the diamond’s potential cut grade.

Example: A round brilliant-cut diamond has an average girdle diameter of 4.55 mm. Its depth from table to culet measures 2.55 mm.

Divide the diamond’s depth by its average girdle diameter: (2.55 ÷ 4.55) = 0.5604

Multiply by 100: 0.5604 × 100 = 56.04%

Round to nearest tenth of a percent = 56.0%

Total depth percentage is useful in determining why a diamond is underweight or overweight in relation to its diameter. Well-proportioned diamonds typically have total depth percentages around 60.0 percent.

Diamonds with total depth percentages below 55.0 percent are generally underweight, with shallow crowns, shallow pavilions, or a combination of both. They might also have large tables or very thin to extremely thin girdles.

Diamonds with total depth percentages of 65.0 percent or more are generally overweight, with steep crown angles, deep pavilions, or both. Excess weight is often located at the girdle, which might range from thick to extremely thick. Some stones might have large tables, shallow crowns, and exceptionally deep pavilions. Others might combine slightly steep crowns with moderately thick girdles and moderately steep pavilions.

It’s important to understand that while a total depth percentage of 60.0 percent suggests good proportions in a diamond, it doesn’t guarantee them. A diamond with a 60.0 total depth percentage might still have negatives like a shallow crown, a very thick girdle, or a deep pavilion.

As you can see from this chart, some total depth percentage (TDP) ranges overlap between grades. For example, a diamond with a total depth percentage of 60.0 percent can fall within any of the GIA cut grades. A total depth percentage of 52.0 percent, however, falls outside the ranges for Excellent, Very Good, and Good and can result in only a Fair or Poor grade. When a diamond’s total depth percentage falls outside any of the top ranges, it can never achieve one of the better cut grades.

Possible Cut Grade(s)

TDP Range

TDP Comments



Extremely shallow crown and/or pavilion, thin girdle

F, P

51.0% to 52.9%

Very shallow crown and/or pavilion, thin girdle

G,F, P

53.0% to 55.9%

Shallow crown and/or pavilion

VG, G, F, P

56.0% to 57.4%

Moderately shallow crown and/or pavilion

EX, VG, G, F, P

57.5% to 63.0%

Generally standard crown, pavilion, and girdle

VG, G, F, P

63.1% to 64.5%

Moderately steep crown and/or pavilion

G, F, P

64.6% to 66.5%

Steep crown and/or pavilion

F, P

66.6% to 70.9%

Very steep crown and/or pavilion, thick girdle



Extremely steep crown and/or pavilion, thick girdle


Diamond Cut: The Shape

CUT of a diamond pertains both to the shape (round, marquise, princess, etc.), and to the make (how well it is cut for proportion and finish). These are the only factors in diamond grading that are controlled by human hands.

First we will talk about the eight major diamond shapes, your first decision when shopping for a diamond. On the next page we will describe a few details about the make of a diamond.

Diamond Shapes



The first step in choosing a diamond often involves selecting your favorite (or her favorite) shape. The Round Brilliant is by far the most popular shape, and it is the most readily available in every possible quality and size.

Contrary to popular belief and perhaps your experience in most stores, fancy-shaped diamonds (as all non-round diamonds are called) are often less expensive than their round brethren... at the wholesale level.

The Princess cut is becoming popular because it is both brilliant and unique. The Princess shape actually saves money for a cutter, since it is closest to the octahedral "habit" of rough diamond crystal, the most common formation of diamond in the rough. (The octahedron is like two pyramids base to base.)

Compared to a Round Brilliant, a cutter can retain more of the original crystal when cutting an octahedron into a Princess shape. The square corners of the rough need to be cut away to create a Round, but they are saved when cutting a Princess.

The more he saves of his original rough crystal, the less the cutter loses on his financial investment in the stone, and therefore you pay less as well.

But many shapes can be beautiful if they are cut well, including the Marquise, Oval, Pear, Radiant, Heart, Emerald and other major shapes. But all fancy shapes have an inherent difference in the physics of light. The longer shapes have a slight "bow tie" effect. This means they have a small zone in the center where light leaks out the bottom, creating a darker area in the shape of a bow tie. This is especially true for the Pear, Oval, Marquise, and Heart shapes.

For ideal proportions that maximize brilliance, fire and sparkle, you can't beat the new Round Brilliant Ideal Cut. The science of cutting a diamond to bring out the full potential of its beauty has developed significantly in the past 10 years. Many diamond cutters now specialize in creating ideal proportions, and such stones have become quite popular. Ideal Cut diamonds command a slight premium because of 1) the extra care and skill needed during cutting, 2) more of the rough is usually cut away, and 3) they are scarce and in high demand.

How to Determine if a Standard Round Brilliant Diamond Is Overweight When Estimating Its Cut Grade?


A standard, one-carat round brilliant diamond typically has an average girdle diameter of 6.50 mm. Diamonds with this diameter that weigh significantly more than one carat, or that weigh one carat but look significantly smaller than 6.50 mm when viewed face-up, are described as “thick” or “heavy” in the trade. These diamonds generally have much more total depth than a well-proportioned diamond should. In other words, they have “hidden weight” and are considered by GIA to be “overweight.”

When a diamond is overweight, it’s heavier than its average girdle diameter might suggest. You can find out if a diamond is overweight by determining its diameter and its carat weight, then comparing the weight to that of a reference diamond with fairly standard proportions.


These diamonds have the same girdle diameters, and they would probably appear to be the same size face-up, but their profiles show very different girdle thicknesses. A thicker girdle (right) is often used to add weight to a fashioned diamond. Photos by Don Mengason.


1. Weigh the diamond and round its weight to the nearest 0.01 ct.

2. Refer to the Diamond Weight Chart to find the estimated weight for a diamond of standard proportions that has the same average girdle diameter as the diamond you are evaluating.

3. Compare the weights. If your diamond weighs more than the example in the chart, calculate the percentage difference between its actual weight and the weight “suggested” by its diameter. To calculate this percentage, take the difference between the diamond’s suggested weight and its actual weight, divide that amount by its suggested weight, then multiply by 100.

For example, according to the Diamond Weight Chart, a diamond with a diameter of 4.02 mm should weigh about 0.24 ct. Your sample diamond’s actual weight is 0.27 ct.

First, subtract the suggested weight from the actual weight.

0.27 − 0.24 = 0.03

Then divide the result by the suggested weight.

0.03 ÷ 0.24 = 0.125

Next, multiply by 100.

0.125  × 100 = 12.5%

Finally, round to the nearest whole percent = 13%

The result tells you this stone is about 13 percent heavier than its diameter suggests. On this basis, the highest cut grade it could receive is Very Good, according to the chart below. Keep in mind that all other factors must be considered to determine if it might receive a lower cut grade.

Diamond Cut Grades and Overweight Percentage

Possible Cut Grade(s)   Percent Overweight

E, VG, G, F, P                <8%

VG, G, F, P                    8% to 16%

G, F, P                            17% to 25%

F, P                                 >25%

You can determine why a diamond is overweight by analyzing its crown height, pavilion depth, and total depth percentage. You should also consider girdle thickness because it is easy to hide weight in an extra-thick girdle.

How to Visually Estimate Fire in a Standard Round Brilliant Diamond?



Strong, concentrated spot lighting will make fire more visible. After you have estimated the brightness and pattern of a diamond, leave it in the tray and switch off the diffused fluorescent light source. Use another spot light source such as an LED flashlight. Tilt the tray to observe the diamond at different angles and make a visual assessment of its fire. Note that observation and evaluation of fire are best done in a slightly darkened room.

In practice, if all other factors are equal, it is more difficult to evaluate differences in fire in stones smaller than a half carat.

Use the following fire-rating guidelines to assign a grade for fire, from Excellent to Poor:

These diamonds display Very Good (left), Fair (middle), and Poor (right) fire. Photos by Eric Welch.

Review your visual estimates for brightness, fire, and pattern. The lowest estimate sets the diamond’s estimated visual cut grade. For example, if you estimated the diamond’s brightness as Very Good and its fire as Very Good, but there was a distracting pattern that only rated Good, your visual cut grade estimate is Good.

How to Visually Estimate the Brightness of a Diamond?



A standardized viewing environment, including controlled lighting and a neutral background, is essential for making consistent comparisons between diamonds. Diamond appearance is very sensitive to changes in lighting conditions, so the same diamond can look quite different when viewed under different types of lighting in different positions. Since diamonds also reflect everything around them, even a grader’s clothing or nearby furniture, it is always best to have a neutral, “uncluttered” environment within which to perform observations.

A mixture of diffuse and “spot” lighting is very helpful for assessing face-up appearance (which includes brightness, fire, and pattern). Fluorescent lighting from an overhead or desk lamp can help evaluate a diamond’s brightness and pattern, but the same light also suppresses its fire. Spot lighting (also known as “point source” lighting) emphasizes a diamond’s fire, but if the lighting is too strong, it can overwhelm everything else and make the diamond appear dark.

The GIA DiamondDock™ provides consistent and repeatable viewing conditions for every diamond cut assessment. It features daylight-equivalent fluorescent light for judging brightness and pattern, as well as an array of light-emitting diodes (LEDs) for judging fire, all housed within neutral gray walls.

However, the DiamondDock™ is not a requirement for discerning differences in diamond appearance. Most diffused light environments that have white-colored ceilings and walls can be used with almost the same results. A standard color-grading box like the Macbeth Judge II, or a common fluorescent desk lamp with D65 fluorescent tubes used in a neutral environment, is a useful alternative for assessing brightness and pattern.

It is also beneficial to use a neutral gray tray to hold the diamonds while observing them. A white background might make a diamond look brighter by hiding or masking areas of light leakage. A black background reveals possible areas of light leakage, but can overemphasize them and make the diamond look too dark. The gray color allows you to see potential areas of light leakage without overemphasizing them.

The following steps outline the procedure for estimating a round brilliant diamond’s brightness using the GIA DiamondDock™ or an equivalent standardized environment.

1. Clean the diamond with a gem cloth.

2. Place the diamond face-up in the gray tray and place the tray in the DiamondDock™ (keeping it near the bottom center of the viewing booth) or a comparable lighting environment. Make sure you’re in a comfortable position and at an optimal viewing distance, usually 12 to 18 inches from the diamond. Turn on only the diffused fluorescent light. Tilt the tray as necessary to observe the diamond at different angles and make a visual assessment of its overall brightness. Remember that you are only assessing the total amount of white light that is returned, not whether that light is in a pleasing pattern. Use the following brightness-rating guidelines:

3. Assign a grade for brightness, from Excellent to Poor.

5 P's of cut appraisal:  
1)--Points (edges and intersections meet precisely which proves that facets have been cut evenly and uniformly)  
2)--Proportions (for the height of most faceted gems the top part represents 1/3 and the bottom part represents 2/3
3)--Polish (a fine polish proves that the previous steps, too, were done properly)  
4)--Positioning (an astute faceter has studied the gem crystal and oriented to conform with optical laws and to minimize the visibility and interference of imperfections and, finally,
5)--Performance (if the vital angles are observed, the stone's facets reflect and refract the maximum amount of external and internal light i.e., it will perform optically - even when tilted or tipped away from normal. This reflective capability contrast with an uninteresting piece of see-through glass when the gem is viewed straight down through the table).
All of a faceter's efforts are focused through HAI to accomplish these Five P's mentioned above.  Here's how it's done.

17.3.1 Common cutting faults
17.3.2 Centre and off centre sawing of diamond octahedral
17.3.3 Steps to evaluate proportions and methods by which determinations can be accomplished
17.3.4 Estimate table size and girdle thickness
17.3.5 Table size and relative depth affect weight and why these and girdle thickness show relative weight retention
17.3.6 Polish, symmetry
17.3.7 Quality of girdle surface
17.3.8 Culet size etc