Cut of Diamonds


Cut refers not to a diamond’s shape (e.g. round, oval, pear, etc.) but to a diamond’s proportions, symmetry and polish. The beauty of a diamond depends more on cut than any other factor. Though extremely difficult to analyze and quantify, diamond cut has three primary effects on appearance: brilliance (the brightness created by the combination of all the white light reflections from the surface and the inside of a polished diamond), fire (the dispersion of light into the colors of the visible spectrum, seen as flashes of color), and scintillation (the flashes of light and dark, or sparkle, when a diamond or light source is moved).

When a diamond is fashioned from a rough stone, the cutter must balance optimal cut (and therefore appearance) against maximum yield (cutting the diamond to maintain as much carat weight from the rough stone as possible). Because many customers are willing to pay more for a larger, fair-cut cut diamond than for a slightly smaller, well-cut diamond, there is pressure on the cutter to sacrifice appearance for weight. This is why the Cut grade is so important; it allows the purchaser to identify those stones that were cut Fair to Poor in an effort to gain carat weight.

GIA Perportions

A too-deep cut diamond (orange) would yield a significantly larger diamond, earning the diamond cutter a larger profit on his investment.

A smaller, well cut diamond (white) may sell for less in total than the larger diamond, but it will command a higher price per carat not only because of its superior appearance, but also due to decreased yield from the rough stone (which therefore makes the diamond more expensive to create).

Questions about cut or other aspects of a diamond? Ask a Beck diamond broker for answers. A consultant will answer any questions you have, and if you like, search for diamonds on your behalf that match your criteria.



Diamond proportion refers to the relationship between the size, shape, and angle of each facet of a diamond. A wide range of combinations are possible, ultimately determining the diamond’s interaction with light.

When light strikes a diamond, approximately 20% immediately reflects off the surface (as glare). Of the 80% that enters, a portion will escape through the bottom of the diamond (where the observer cannot appreciate it). A well proportioned diamond will have each facet properly placed and angled so as to maximize the amount of light that reflects back out of the crown (top) of the diamond, to the eye of the observer. This reflected light is perceived as scintillation, fire and brilliance.

In the diagram below, three common light patterns are shown. When light meets any facet of a diamond, it will either reflect (bounce back) or refract (bend while passing through the facet). The angle that the light hits the facet determines whether the majority of light reflects or refracts, which is why cut is so important.

Shallow Diamond Cut If the diamond cut is too shallow, entering light strikes the pavilion facet at a low angle and passes through the facet (refracts), escaping through the bottom of the diamond.
Deep Diamond Cut If the diamond cut is too deep, entering light strikes the first pavilion facet at an angle sharp enough to reflect to the second pavilion. But the light strikes the second pavilion at too low an angle, causing the light to refract (pass through the facet), escaping through the bottom of the diamond.
Correct Diamond Cut In a well cut diamond, the light strikes each pavilion facet at an angle which allows most of the light to reflect back to the crown (top). As it passes through the crown facets at a low angle, the light refracts upon exit. In this case, refraction is a good thing, as the bent light travels to the observer’s eye and is perceived as a lively fire.


Depth refers to the distance between the culet and the table when the diamond is viewed from the side. Diamond depth is expressed in millimeters, and is given as the third number under “measurements” for every Beck Diamond (the first two numbers are length and width).

The depth % of a diamond is calculated by dividing the depth by the width of the diamond. So, if a diamond is 3 mm in depth, and 4.5 mm in width, its depth % is 66.7. The lower the depth %, the larger a diamond of a given carat weight will appear (since most of the diamond’s size is in its width vs. in its depth).


The table % of a diamond is calculated by dividing the width of the table facet by the width of the diamond. So, if the table facet is 3 mm wide, and the diamond is 4.5 mm wide, its table % is 66.7.

A note of caution:

A diamond should not be chosen or rejected based solely on depth % or table %. Because the overall Cut grade already incorporates both factors, it should be used as the primary determinant when choosing a diamond. When comparing two diamonds of equal Cut grade, depth % and table % can then be used as further refinements, especially (in the case of depth %) if one is concerned about how large the diamond will appear.


GIA Diamond Proportion Diagram

Every Lumera Diamond comes with a GIA  Grading Report or Dossier certificate, most of which include a proportion diagram. The proportion diagram is a graphic representation of the diamond’s actual proportions.

The proportion diagram shows the diamond’s girdle size, culet size, table and depth percentages, as well as other measurements, such as the crown and pavillion angles.

Each angle and dimension is measured electronically using a light scanner. The proportion diagram is a “fingerprint” of your diamond.

GIA Diamond Cut Grades
Excellent, Very Good, and Good GIA Cut Grades


Evaluating the combined effects of facet shapes and angles, girdle width, culet size, polish and symmetry on the overall appearance of a diamond is a daunting task, even for professional gemologists. Fortunately, all of these factors have already been taken into consideration when calculating the diamond’s Cut grade.

Because Cut grade provides a single rating which weighs the combined impact of all of the factors listed above on a diamond’s visual performance, it is a simple yet vital tool in evaluating a diamond.

Cut grade should be a primary consideration when evaluating a diamond. Only when comparing two diamonds of identical Cut grade should the individual components of Cut (such as girdle width, symmetry, polish, depth%, table %, and culet size) be used as further refinements or tie breakers.

A note of caution:

Unlike the other “Cs” (caret weight, colur, and clarity), the various Cut grades in existence today were not originated by GIA, and are not uniformly applied. In fact, GIA has only been assigning cut grades since 2005, and only to round diamonds.

Even though retailers use common terms to describe Cut (such as Excellent, Very Good, Fair, Poor) the terms are not uniformly defined or applied. In fact, a diamond seller may assign any cut grade they choose, based on any set of factors they wish. One retailer will use terms such as “Signature Ideal”, “Ideal”, and “Excellent”; while another uses “Ideal” to describe all three, and another uses “Excellent” for all. Be cautious when comparing cut grades from different sources, as they are most likely inconsistent.

To avoid confusion or misrepresentation of any diamond, Lumera uses the same cut grades as employed by GIA, and does not engage in “cut inflation” by creating other grades (“signature”, “super ideal”, etc.).

Also, be aware of diamond sellers who assign their own cut grades in place of what GIA has already assigned to a particular diamond. Many popular websites and retail stores display their own, more generous Cut rating, in place of the GIA grade.

At Lumera, the GIA cut grade is always shown. For diamonds without a GIA cut grade (because the diamond was certified before GIA began assigning cut grades in 2005, or is a shape that GIA does not currently assign cut grades to), Lumera will apply its own cut grade according to a proprietary formula. Like GIA, the Lumera cut grade is based on a variety of factors and is conservatively applied. In fact, when comparing our cut grade to that assigned by GIA, the Lumera grade is equal to or lower than GIA 90% of the time.

Cut grade is assigned by the GIA (or Lumera itself when there is no GIA grade), using the following scale:


Maximum fire and brilliance. Reflects nearly all of the light that enters the diamond, creating exceptional sparkle and life.

Very Good

Properly reflects most of the light that enters the diamond, producing superior fire and brilliance. Under normal lighting conditions, appears very similar to Excellent Cut, but for a lower price.


Reflects a majority of the light that enters the diamond, for an above average appearance. An excellent value compared to higher cut grades.


Allows much of the light entering the diamond to escape from the sides or bottom, reducing perceived fire and brilliance. More acceptable in diamonds of less than .75 carats, where differences in sparkle are more difficult to perceive.


Allows most of the light entering the diamond to escape from the sides or bottom. The diamond may appear noticeably dull and lifeless, even to an untrained eye. Lumera does not offer Poor cut diamonds.


Beck Diamonds Buying Tip:

Cut grade is the most important factor in determining the overall appearance of a diamond, because a poorly cut diamond will seem dull even with excellent clarity and colour. Conversely, a well cut diamond can have a slightly lower color (G-H) or clarity (SI1-SI2) and still look quite beautiful, due to its superior ability to create sparkle and brilliance.

For superior brilliance, choose a diamond with a Cut grade of Very Good or Excellent for round diamonds, and Good or better in fancy shape diamonds. When choosing a diamond in this range, make sure its Symmetry and Polish are Very Good or Excellent, so that the impact of the above average Cut is not obscured.

For those on a budget, primarily concerned with size, a diamond of Fair – Good cut may be an acceptable choice, especially in fancy shapes. While the diamond will lack the scintillation and brilliance of a well cut diamond, it will allow a significant increase in size for the same price.

Avoid Poor cut diamonds, even if size is the primary concern. Most find these diamonds to be an unacceptable trade off, despite the lower price. Lumera does not offer Poor cut diamonds.