Diamond Buying and the Four Cs, Part 3: Evaluating Diamond Cuts


Summary
Of the Four Cs of gem grading, cut is perhaps the most complex. As a result, consumers might ignore this property and thus take it for granted. However, lapidaries must carefully consider diamond cuts in combination with the other gem quality factors — carat, color, and clarity.

In this third of a 4-part series of articles, we’ll introduce diamond buyers to the basic terminology of cutting and explain how professionals grade cut quality.

Reading time: 14 min 37 sec
diamond cuts - dispersion in a diamond engagement ring
Faceted diamond showing dispersion. Photo by James St. John. Licensed under CC By 2.0.
faceted diamond - diamond cuts
Diamond. Photo by Open School of Diamond Gemology. Licensed under CC By 3.0.

Choosing and executing diamond cuts successfully takes an understanding of light and angles. No wonder famed diamond cutter Marcel Tolkowsky was also a mathematician! Cutting is critical to the optical performance of diamonds. Unfortunately, when most people view a diamond, carat size usually makes the biggest impression. In reality, cut greatly affects how large a diamond appears and the light pattern it shows. It also largely determines its brilliance — how much light it returns — and dispersion — how much internal “fire” it displays. If poorly executed, the cut can ruin these most prized aspects of diamonds – not something you want in an engagement ring!

Diamond Cut Grades

For round brilliant diamonds, cut grades are as follows:

  • Excellent
  • Very good
  • Good
  • Fair
  • Poor

Lasers do most of the cutting nowadays, so most diamonds should fall in the Very Good to Excellent range.

The only reason diamonds ever have a less than Excellent cut is because the diamond cutter is trying to save weight. Diamond cutting is usually a compromise between saving weight and maintaining good brightness, pattern, and symmetry.

A diamond of a larger carat size simply commands a higher price despite having a somewhat lower cut grade. In other words, although a diamond with a lower cut grade will go down slightly in price, a larger carat size will more than make up that deficit. However, if the cut is so poor that the diamond appears clumsy, its value would be diminished.

So, buyer beware. If you purchase a stone with a less than Excellent cut grade, you may have bought a thick stone that hides weight but doesn’t appear larger compared to smaller or same-sized stones. Furthermore, a badly cut stone can also have less brilliance and fire than a comparable well-cut stone.

fair grade - diamond cuts
An example of a diamond with a cut grade of Fair and poor scintillation. © International Gem Society.

The Anatomy of a Diamond

Diamond graders evaluate round brilliant cuts based on how closely a diamond comes to having ideal proportions and, thus, showing maximum brilliance and fire. These proportions depend on the table size, the crown angle, girdle thickness, pavilion angle, culet, and many other factors. These individual factors can receive cut grades, too.

Here is an overview of the anatomy of a faceted diamond and the grading standards for diamond cuts.

Table

The table is the largest facet on a diamond. Most of the light enters the stone and bounces back to the viewer through this facet. It should be fairly large, otherwise the diamond will look small and boring. If it’s too large, however, the diamond will have no fire due to the smaller crown main facets.

A diamond should have a table size roughly between 52-62% of the diamond width, also known as the average girdle diameter. Within this range, table size is mostly a matter of personal taste, a trade-off between brilliance and fire.

table size chart - diamond cuts
Diamond table sizes of 54% (left), 60% (center), and 66% (right) of diamond width. Images by Open School of Diamond Gemology. Licensed under CC By 3.0.

Crown

The ring of facets around the table, the crown affects how much dispersion or fire the diamond shows. Size as well as the angle of the crown facets matter here. Cut the crown angle shallow and the diamond will have excellent brilliance but no fire. Cut the crown angle steep and the diamond will have excellent fire but no brilliance.

Steep crown angles also hide a lot of weight without making the diamond appear any bigger.

Cutters will often choose a steep or shallow crown angle depending on whether they’re working with longer or wider rough.

Ideally, the crown angle should be between 31.5-36.5°. As with table size and crown facet size, the crown angle is a trade-off between brilliance and fire. In order for the stone to have excellent symmetry, it must have the same crown angles all around. If the facets have differing crown angles, the table will appear slanted. The crown height percentage, the proportion of the crown in comparison to the total depth of the diamond, should be 12.5-17%.

crown angles - diamond cuts
Crown angles of 30° (left) and 45° (right). Images by Open School of Diamond Gemology. Licensed under CC By 3.0.

Girdle

The thin belt around the widest part of the diamond, the girdle doesn’t cover much surface area. Nevertheless, it’s extremely important to the overall proportion of the diamond. If cut too thin, the girdle becomes a knife-edge and will chip easily. If cut too thick, the diamond gains weight without looking bigger. (Since the girdle wraps around the widest part of the diamond, any increase in thickness increases the carat weight dramatically).

girdle width comparison chart - diamond cuts
Girdle widths compared. From top to bottom: very thin, thin, medium, thick, and very thick. Images by Open School of Diamond Gemology. Licensed under CC By 3.0.

Ideally, the girdle thickness should be between 3 to 3.5% of the total depth percentage (more on this later).

girdle width - diamond cuts
The thicker the girdle the larger the loss of light. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.
Bruted Girdles

The girdle can be bruted, which means rounded by abrading against another diamond. Bruted girdles have a frosty look and a grainy, sugar-like texture. Unique to diamonds, this “bearded” texture helps separate natural diamonds from imitations.

Some cutters believe that polished or faceted girdles reflect back inside the stone, so they will leave stones of K color or lower bruted.

Bruted girdles do have a drawback. Feathers or tiny cracks will sometimes form along the girdle during the rounding and abrading process.

bruted girdle comparison - diamond cuts
The girdle on the left is grained more coarsely and has fringes extending outside the girdle. This gives the girdle a “bearded” look. In contrast, the girdle on the right has a fine grain and looks even. Photos by Open School of Diamond Gemology. Licensed under CC By 3.0.
Polished Girdles

A clear and smooth polished girdle becomes one large facet that allows viewers to see into the stone.

Faceted Girdles

Faceted girdles have numerous facets all along the edge. Nowadays, most diamonds, especially high-quality stones, have faceted girdles.

Faceted diamonds are first bruted. Any tiny feathers or cracks formed during the bruting process are then removed during faceting. Thus, faceted girdles take more time to create and may result in a minute loss of weight. However, they tend to give the diamond a cleaner, more finished look.

The girdle finish itself isn’t usually noticeable to the naked eye unless the diamond is extremely large or the girdle unusually thick.

faceted girdle - diamond cuts
Faceted girdle. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.
Painting and Digging Out

Sometimes, the girdle will be uneven, with some portions thicker than others. Gem cutters might deliberately cut the girdle this way in order to save weight, a method called painting. In other cases, they may cut the girdle extremely thick to later cut away parts that contain unattractive inclusions. This method is called digging out. Both create an unevenly scalloped girdle and can change the face-up pattern of the diamond. Severe painting causes the diamond to have larger flashes of white. This makes the pattern of the diamond appear bland and uneven. Severe digging out causes some of the lower crown facets to look dark. This makes the diamond appear smaller.

The Gemological Institute of America (GIA) grades for painting and digging out are: Negligible, Moderate, Significant, and Severe.

painting and digging out girdle - diamond cuts
An irregular, uneven girdle. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.
Girdle Inscriptions

Girdles have another important function. If a diamond has an identification number, it’s laser inscribed here. The inscription is like the diamond’s Social Security number. It identifies and matches the diamond with a specific grading report.

Culet

The term “culet” may refer to a point or a small facet at the bottom of a diamond. A faceted culet helps prevent chipping and forming unattractive feathers. Not all diamonds have culets. (Round brilliant diamonds with culets have fifty-eight instead of fifty-seven facets).

Take special care of loose, unmounted diamonds without culets. Most jewelry mountings will protect the diamond’s culet from chipping.

Wittelsbach Diamond - diamond cuts
Before being recut and renamed in 2010, the Wittelsbach Diamond, shown here, had an enormous culet visible through its table. Photo by Physolamuse. Public Domain.

Graders classify culet sizes according to visibility. They are: None, Very Small, Small, Medium, Slightly Large, Large, Very Large and Extremely Large. Viewing “Very Small,” “Small,” and “Medium” culets requires 10X magnification. You can see “Large” and “Very Large” culets with the naked eye.

Culet sizes of None to Small are preferred. They will receive a GIA Excellent cut grade. A culet size of Medium will receive a GIA Very Good cut grade. Any culet larger than Medium will be reflected as an unattractive dot at the center of the table. Extremely Large culets will appear as a dark circle.

culet symmetry - diamond cuts
Uneven pavilion facets can cause the culet to be skewed. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.

Total Depth Percentage

The total depth percentage tells if the diamond is over or under weight. Graders calculate it by dividing the average girdle diameter (the width of the diamond) by the table to culet length (or height) of the diamond and multiplying it by one hundred. Ideally, the total depth percentage should range from 57.5 to 63%. A diamond with a total depth percentage under 55% may have shallow crowns, shallow pavilions, thin girdles, or a combination of these. In contrast, a diamond with a total depth percentage over 65% may have steep crowns, steep pavilions, thick girdles, or a combination of these.

Pavilion Depth Percentage

The pavilion reflects the table. So, when you look into the diamond, you’ll see a gray octagon. This is the table’s reflection in the pavilion. The pavilion depth percentage should range from 41-48%.

Pavilions shallower than 41% will make the reflection pattern small and fragmented. This will give the diamond a messy look. Furthermore, the girdle reflection will appear as a “fish eye,” an unattractive gray ring around the table facet. Especially common in stones that combine shallow pavilions with large tables, fish eyes make diamonds look dull.

Pavilions deeper than 48% will cause the stone to look dark. If the pavilion becomes deeper than 50%, the entire table will look dark. This effect is called a “nail head.” Note: this differs from nail head inclusions found in some synthetic gems.

nail head and fish eye effect - diamond cuts
Nail heads and fish eyes are characteristic of poorly cut diamonds. © International Gem Society.

Pavilion Angle

The pavilion angle should range between 37.4 to 44°. Shallow angles might cause fish eyes, while steep angles make the stone look dark.

bulging pavilion - diamond cuts
This diamond with an extremely bulging pavilion is 15% overweight. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.

Polish

A diamond’s polish covers how carefully the cutter finished the stone’s facets. The GIA rankings are: Excellent, Very Good, Good, Fair, and Poor.

polish - diamond cuts
Diamonds are polished on a spinning lap coated with diamond dust. Photo by Andere Andre. Public Domain.

Most stones will have transparent polish lines from the polishing process. If these aren’t too obvious and don’t interfere with the visual appeal of the stone at 10x magnification, the diamond will receive an Excellent polish grade. In contrast, numerous or white polish lines and burns will earn a lower polish grade.

A stone’s polish isn’t an integral part of the stone. An expert re-polishing can easily remove unattractive marks. However, as long as the bad polish remains, it will make the stone appear dim and hazy, thus significantly impacting the diamond’s appeal.

polish lines - diamond cuts
Polish lines. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.

Symmetry

Symmetry, on the other hand, is an integral part of the stone. Graders assess whether facets of the same type all have the same size as well as proper angles and alignment.

transposed facets - diamond cuts
Transposed facets don’t align, which impacts diamond symmetry. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.

For example, bezel or break facets (the facets on the crown touching the girdle) that don’t align with the pavilion facets (the long facets on the pavilion touching the girdle) look very unattractive. Uneven crown angles make the table off-kilter, and uneven pavilion facets create an off-center culet. The girdle should wrap around the diamond evenly, instead of rippling up and down. The face-up shape of the diamond should be round.

irregular table edges - diamond cuts
Irregular table edges. Images by Open School of Diamond Gemology. Licensed under CC By 3.0.

These are just a few of the many aspects diamond graders must consider when evaluating symmetry.

tilted table - diamond cuts
Table tilted towards girdle plane. Image by Open School of Diamond Gemology. Licensed under CC By 3.0.
“Hearts and Arrows”

Many consumers think the diamond cut marketed as “Hearts and Arrows” must have an Excellent cut. Actually, this cut only indicates perfect symmetry. A “Hearts and Arrows” diamond shows an arrow pattern through the table and a heart pattern through the pavilion. It will always have Excellent symmetry. However, it won’t always be Excellent in other aspects of the cut grade.

This cut has become so popular that “Hearts and Arrows” diamonds can sometimes cost more than comparable diamonds with other cuts, especially when purchased from companies that specialize in this cut. Highly symmetrical diamonds do take more time to cut and waste more diamond rough weight than other diamonds. This contributes to their price premium.

"Hearts and Arrows" diamond engagement ring - diamond cuts
The arrow pattern is visible from the front of the diamond in this engagement ring. Photo by James Champagne. Used with permission.

End Result: Brightness, Pattern, and Fire

The art of judging diamond cuts isn’t merely measuring and calculating angles and percentages to see if they match the specified parameters. A diamond may receive Excellent grades for the individual categories of table size, crown angle, and total depth percentage but only earn a Very Good or Good overall GIA cut grade. It may be that these properties — individually excellent — just don’t work well together.

A diamond’s overall cut grade is only as high as the lowest grade in any of the cut subcategories.

The end result of all these cut parameters should be a stone that has amazing brightness, pattern, and fire. A stone with good brightness should (of course) be bright, especially in the table around the culet. It should have its light distributed evenly across the crown. A diamond with good pattern has even and strong contrast between light and dark. A stone with good fire shows flashes across most of the crown facets. The fire should display a variety of colors, but red counts as the most valued.

dispersion display in engagement ring - diamond cuts
This faceted diamond displays considerable fire or dispersion. Photo by James St. John. Licensed under CC By 2.0.

Triple Excellent

A common trade term, “triple excellent” means the cut, symmetry, and polish all rank as Excellent according to GIA standards. However, “triple excellent” stones are fairly standard. This designation shouldn’t affect prices drastically.

Grading Fancy Diamond Cuts

Grading fancy diamond cuts (as opposed to round brilliants) involves much looser and more subjective parameters. Round brilliants gain value for brilliance and fire, which require exacting research and precision cutting to produce. Fancy cut diamonds, on the other hand, gain value primarily for their shape appeal. Pattern and brilliance become secondary concerns.

To learn more about the many varieties of fancy cuts, read this introduction and the guide to diamond shape, size and value.

Judging Diamond Cut Quality Online

Because the cut is so important to a diamond’s beauty, it’s essential to see a diamond perform before buying it. While certificates are great for weeding out diamonds with poor performance, they can’t tell you the difference between an Excellent cut with mediocre performance and one that’s dazzling.

In order to judge diamond cut quality online, you’ll need to see a close-up video. Both James Allen and Blue Nile offer magnified 360° videos of each of their thousands of loose diamonds, giving you the ability to judge the performance for yourself. Better yet, they offer plenty of ring designs to choose from, letting you find your perfect match.

About the author
Phoebe Shang, GG
A gem lover and writer, Phoebe holds a graduate gemologist degree from the Gemological Institute of America and masters in writing from Columbia University. She got her start in gemology translating and editing Colored Stone and Mineral Highlights for a professor based in Shanghai. Whether in LA, Taipei, or New York, Phoebe spends her time searching for gems to design and being lost in good books.
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