Back to: Diamond Grading Basics
4Cs of Diamond Quality: Diamond Cut Grading by GIA. As the creator of the 4Cs of diamond quality,
GIA established the universal language for communicating a diamond’s value.
Diamonds are renowned for their ability to transmit light and sparkle intensely. Learn about the most complex of the 4Cs, diamond cut, with this one-minute video.
A diamond’s cut is what unleashes its light. Precise artistry and workmanship are required to fashion a stone so its proportions, symmetry, and polish deliver the magnificent return of light only possible in a diamond.
The GIA Cut Grading System for the standard round brilliant diamond evaluates seven components. Brightness, fire, and scintillation consider the diamond’s overall face-up appearance while weight ratio, durability, polish, and symmetry assess a diamond’s design and craftsmanship. Interested in diamond cuts?
Popular cuts of diamonds can range from the princess cut diamond to the cushion cut diamond to the emerald cut diamond.
Cut is the human contribution to a diamond’s beauty. A diamond cut is a style or design guide used when shaping a diamond for polishing such as the brilliant cut. Cut does not refer to the shape but refers to the symmetry, proportioning and polish of a diamond. Diamond cutters must consider several factors, such as the shape and size of the rough crystal when choosing a cut.
Based on scientific formulas, a well-cut diamond will internally reflect light from one mirror-like facet to another and disperse and reflect it through the top of the stone. This results in a display of brilliance and fire, thereby placing well-cut diamonds higher on the Diamond Quality Pyramid than deep or shallow-cut diamonds. Diamonds that are cut too deep or too shallow lose or leak light through the side or bottom, resulting in less brilliance and ultimately, value.
Proportion
A well-cut diamond reflects light back to the eye evenly in the face-up position, with no dark areas. Dark or “dead” areas are due to poor cutting, and should not be confused with the faint “bow-ties” which are typical of fancy shapes; in the case of fancy shapes, bow-ties are where the main brilliance and life of the diamond are most apparent
.Ideal Cut Proportions
When a diamond is well-cut (either a fine cut or an Ideal Cut) light enters through the table and travels all the way to the pavilion where it reflects from one side to the other – intensifying in the mirror-like facets as it travels – before reflecting back out of the diamond through the table and to the observer’s eye.
Brilliance
This brightness that seems to come from the heart of a diamond is known as brilliance.
It is the effect that makes diamonds unique among all other gemstones. While other gemstones also display brilliance, none have the power to equal the extent of diamond’s light-reflecting power. In a poorly cut diamond, however, the light that enters through the table reaches the pavilion facets and then “leaks” out from the sides or bottom of the diamond rather reflecting back to the eye.
Symmetry
Symmetry refers to how precisely the various facets of a diamond align and intersect. This can include extra or misshapen facets, off centre culets and tables, and wavy girdles. A diamond with poor symmetry may misdirect light that travels into the diamond, sending it off at slightly wrong angles, and thereby reducing the diamond’s brilliance. Often, a diamond cutter will purposefully allow a minor reduction in symmetry as a way of preventing a defect present in the rough stone from being retained as part of the finished diamond.
For diamonds with a symmetry grade of Excellent to Good, symmetry should not be used as a primary factor in choosing a diamond, since each of these grades is possible in diamonds of exceptional appearance.
Symmetry is more important in diamonds of VVS2 Clarity and higher since the very subtle defects produced by Fair or Poor symmetry (which can resemble pinpoint inclusions) would compromise the diamond’s otherwise flawless appearance.
Despite its modest impact on appearance, symmetry has a significant impact on the price; a diamond with Excellent Symmetry and Polish may be priced 10%-15% higher than a diamond with Good Symmetry and Polish. This premium has more to do with the consumer’s perceived value of “excellent” grades, than the actual effect on a diamond’s appearance.
Because diamonds with Poor symmetry have defects visible to the naked eye, these diamonds should be avoided in all cases.
Polish
Polish refers to the degree of smoothness of each facet of a diamond as measured by a gemmologist. When a diamond is cut and polished, microscopic surface defects may be created by the polishing wheel as it drags tiny dislodged crystals across the diamond’s surface. Depending on the severity, these defects may disrupt light patterns as the light rays enter and exit the diamond. While inclusions occur naturally as the diamond forms, polish marks are the result of human error. Nearly every diamond has some sort of flaw due to polish. But unlike symmetry, many of these flaws are so tiny that they cannot be seen with the naked eye.
Some polishing marks are so faint that gemologists can only find them under higher magnification and at a certain angle. The most common polish marks are:
- Scratch: A transparent-looking white line on the diamond’s surface.
- Nick: A small notch on a facet or girdle junction.
- Abrasion: A collection of nicks at facet junctions.
- Pit: A small opening in the surface, usually due to inclusions falling out during polishing.
- Lizard skin: A bumpy looking texture.
- Rough girdle: A girdle that has been left unpolished.
- Burn marks: A whitish haze on a diamond’s surface, caused by high temperatures from a polishing wheel.
For diamonds with a polish grade of Excellent to Good, any polishing defects are not visible to the naked eye and should have no impact on the diamond’s overall appearance.
For diamonds with clarity grades of I1 or lower, even a polish grade of Fair is acceptable, since these diamonds already possess internal inclusions that are visible to the naked eye, making any polish markings less relevant.
Finish: refers to the qualities imparted to a diamond by the skill of the diamond cutter. The term “finish” covers every aspect of a diamond’s appearance that is not a result of the diamond’s inherent nature when it comes out of the ground. The execution of the diamond’s design, the precision of its cutting details, and the quality of its polish are all a consideration when a gemologist is grading “finish”.
If you examine a diamond’s grading report, you will see its finish graded according to two separate categories: Polish & Symmetry.
Diamond Grade
Usually, a diamond will be classified as Ideal Cut only if it meets the highest qualifications established by the American Gem Society’s Gemological Laboratories. These are strict standards set for proportion, symmetry and polish. Be wary of retailers who call a stone Ideal without a Diamond Quality Report (DQD) from a lab to back up their claim.
Diamond Grades: Excellent, Very Good, Good, Fair, and Poor.
Round Brilliant Cut
The three main sections of the Round Brilliant Cut
The Crown
In a round diamond, in order for the crown to provide sufficient fire, the bezel facets should be cut within a specific range of angles (usually between 33° and 35° degrees) However, this angle is merely a guideline, not a hard-and-fast rule that must be adhered to in every case.
At the outer points of this range, you might find diamonds with crown angles of as little as 31.5° degrees and as much as 35.9° degrees which are still very attractive. These angles do not affect the diamond’s table percentage in any way. It is possible for a diamond’s crown to have any combination of crown angles and table size that a cutter desires.
The Girdle
The girdle is the outer edge of the diamond, where the crown (top) meets the pavilion (bottom).
A girdle may be faceted (a series of tiny polished sides going around the diamond), bruted (a single continuous unpolished surface going around the diamond; no longer common), or polished (a bruted girdle that has been polished smooth). Whether a girdle is faceted, bruted, or polished usually has no impact on the appearance or value of the diamond.
The girdle is described according to its width. Often, the width of the girdle varies at different points around the diamond and is quoted in a range designating the thinnest and thickest point along the girdle.
(“Thin – Medium” means the diamond’s girdle varies in width from thin at the narrowest point to medium at the widest point).
The Pavilion
The most overlooked part of the diamond
The pavilion is the part of the diamond just below the girdle, it is easy to see why people often neglect to consider its contribution to a diamond’s beauty; when a diamond is set, typically only the crown stands out prominently, and the girdle and pavilion are hidden beneath prongs. However, it is this hidden part of the diamond that is the key to brilliance. The secret is in the pavilion angles, which, in a round diamond, should typically be between 40.5° and 41.5° degrees.
Unlike crown angles, variations in pavilion angle can produce drastic and, in some cases, devastating differences in the appearance of a diamond. However, even here, the rules allow some leeway for a cutter who is trying to perfect his art. For example, according to AGS, a round diamond may have a 40.25° degree pavilion and still be considered an “ideal cut”.
A fisheye is the reflection of a diamond’s girdle that is visible through the table; it appears as a faint white ring in the centre of the diamond (the effect looks like a fish’s eye, hence its name) Even though we generally prefer to avoid these diamonds, we want to emphasize that having 40.25° degree pavilion angle does not necessarily mean a diamond will display a fisheye or that it will be “ugly” in any way.
Diamond cutting is an art, not a science, and cutters learn, through years of experience, to consider all aspects of their finished product. While a certain pavilion angle may not achieve the proper look in combination with one set of crown angles and table sizes, when the same pavilion angle is paired with a different arrangement of crown facets, the result can be stunning. That said, there are still variations on pavilion angle that are definitely unacceptable.
Cutting a pavilion of 40° degrees (just ½ degree shallower than the standard) produces what is considered industry-wide to be a very clear and very undesirable fish eye. And on the deeper side of the spectrum, while a 42° degree angle can still produce a beautiful diamond, anything deeper than that will create a “nailhead” a dark, lifeless spot in the centre of the stone caused when the light entering the diamond is lost in the bottom of the pavilion and fails to reflect back through the crown to your eyes.
Facets
Facets are the surfaces of a diamond that can be polished. These facets can then refract the light within the diamond and give off the coloured light known as Fire.
| FACET NAME | AMOUNT |
| Table | 1 |
| Bezel or Kite | 8 |
| Star | 8 |
| Upper Girdle | 16 |
| Lower Girdle | 16 |
| Pavilion Main | 8 |
| Culet | 1 |
There are 58 Facets in total
The “table” is the largest and top-most facet on the diamond’s crown. The table percentage is the value, which represents how the diameter of the table facet compares to the diameter of the entire diamond. So, a diamond with a 53% table has a table, which is 53% as wide as the diamond’s outline. For the round diamond, gemmologists calculate table percentage by dividing the diameter of the table, which is measured in millimetres (this millimetre measurement does not appear on diamond grading reports) by the average girdle diameter (an average of the first 2 millimetre measurements on the top left-hand side of a diamond grading report). For a fancy shape diamond, table percentage is calculated by dividing the width of the table, at the widest part of the diamond, by the millimetre width of the entire stone (this total width measurement is the second of the 3-millimetre values in the top left-hand corner of the diamond grading report).
Contrary to popular misconception, having a small table percentage (53% to 57%) does not make a round diamond any more brilliant than a diamond with a larger table. Rather, the table percentage plays a far subtler role in the interaction between a particular diamond and the visible light surrounding it. It is meant to reflect and return white light to the eye, creating those quick flashes of light you see as a person tilts the diamond back and forth during normal movement.
These quick flashes of light are known as Scintillation.
Arranged around the table are several smaller facets (bezel and star facets) angled downward at varying degrees. These facets and the angles at which they are cut have been skillfully designed to break up white light as it hits the surface, separating it into its component spectral colours red, blue, and green. This effect, which appears as a play of small flashes of colour across the surface of the diamond as it is tilted, is what we refer to as the diamond’s dispersion (“fire”). This play of colour should not be confused with a diamond’s natural body colour (normally white, though sometimes yellow, brown, pink or blue in the case of fancy colour diamonds) which is uniform throughout the entire diamond, regardless of whether it is being tilted or not.
Total Depth Percentage
The total depth percentage of a round cut diamond is the table-to-culet depth.
With round brilliant cut diamonds, a total depth percentage in the upper 50% or lower 60% results in a beautiful diamond. However, with a princess and radiant cuts, this depth would create a diamond that looked like glass – not at all like the diamond you would expect. This is because more of the weight and more of the depth, in a fancy shape must be dedicated to the pavilion in order to create the proper angles for light to be reflected back to the eye. Therefore, in a princess or radiant cut with a proper crown, total depth of 70% or more is not uncommon for well-cut diamonds.Total Depth Percentage
These two shapes generally require the greatest depths of all the fancy cuts. Despite the fact that some shapes require greater depth than others, you should not obsess too much over total depth percentage. Remember that, in the end, brilliance is based less on the total depth of a diamond than on how that depth is distributed, and on how the depth interacts with the particular crown and pavilion angles of the diamond. Predecessors of the modern round brilliant, such as the European or Old Mine Cut, were fairly deep stones with very small tables, large culets and short pavilion facets; there was no single standard way
However, in 1919, diamond cutter Marcel Tolkowsky published a doctoral dissertation that would change all of that. Using only his own visual assessments of different variations of diamond cuts; he presented a theory about the cutting angles, which would create the most proportionate balance of brilliance and dispersion in a gem-quality diamond. Tolkowsky’s measurement for achieving this balance was exact: a 34½° crown angle with a 53% table, which created a 16.2% crown height; and 40¾° pavilion angle combined with a 43.1% pavilion depth. Despite Tolkowsky’s theories, opinion regarding diamond proportions was not unanimous.
Tolkowsky’s measurements were eagerly adopted and adhered to by the AGS. However, by the beginning of the 1950s, a backlash had begun and diamond cutters increasingly moved away from the ideal cut and toward diamonds with shallower crown angles – angles as low as 32½° and larger tables of 60% and even 64% or 65%. Many went so far as to argue that the proportions of the ideal cut created an inherent over-abundance of dispersion (“fire”), which distracted from the diamond’s brilliance. As proof that the ideal cut was not the absolute embodiment of perfection, they pointed to Eastern cultures, which actually considered larger tables more beautiful than the smaller ones, which typified an ideal cut. Even those who embraced the Ideal Cut realized the impracticality of cutting diamonds to such a specific set of parameters and soon modified its definition by expanding the acceptable table size from Tolkowsky’s original 53% up to nearly 58%.