The cut of a diamond determines its brilliance. 

There is no single measurement of a diamond that defines its cut, but rather a collection of measurements and observations that determine the relationship between a diamond’s light performance, dimensions, and finish. 

Most gemologists consider cut the most important diamond characteristic because even if a diamond has perfect color and clarity, a diamond with a poor cut will have dulled brilliance. 

The diamond structure

A diamond is made up of two key sections, the Crown and the Pavilion. 

Their structure and relationship to each other in the form of table and depth percentages have the most significant impact on the diamond’s sparkle. 

The brilliant round cut has been used as an example because, with most other diamond shapes (“fancy cut” diamonds), the cut grading is more complicated (hence why their cut grading does not appear on certificates such as GIA).

Diameter

The width of the diamond as measured through the girdle.

Table 

The most significant facet of a gemstone. 

Crown 

The top portion of a diamond extending from the girdle to the table. 

Girdle

The intersection of the crown and pavilion, which defines the perimeter of the diamond. 

Pavilion

The bottom portion of a diamond, extending from the girdle to the culet. 

Culet

The facet at the tip of a gemstone. 

The preferred culet is not visible with the unaided eye (graded “none” or “small”). 

Depth

The height of a gemstone measured from the culet to the table.

Percentage ratios 

The cut is mainly influenced by the harmony between the table and depth percentages and crown and pavilion angles, either causing the diamond to dissipate light (poor cut) or optimally refract and reflect light (excellent cut). 

The table and depth percentages are calculated as follows:

Depth Percentage

The higher the number, the deeper the stone. The lower the number, the shallower the stone.

Table Percentage

The higher the number, the bigger the table looks. 

The lower the number, the smaller the table looks. 

Table and depth percentages affect how light travels within the diamond and impacts a stone’s brilliance. 

If a cut is too shallow, light escapes out from the sides, and the diamond loses brilliance. 

If the cut is too deep, light is lost from the bottom, and the diamond appears dull or dark.

The effects of light  

The science behind a diamond’s brilliance depends on its excellent ability to bend, slow and direct light as it passes through.

The cut of a diamond determines how well the diamond can re-direct the light back through the surface of the diamond. 

Light traveling at about 186,000 miles per second, when passing through a diamond, is reduced to about 77,000 miles per second, close to the maximum for any transparent substance.

The cut is designed to create maximum brilliance and scintillation in the most symmetrically pleasing manner. 

The crown’s job is to catch the light and generate scintillation by reflection, while the pavilion is responsible for both brilliance and scintillation. 

A gem cut with a smooth, cone-shaped pavilion could still display brilliance but would lack scintillation. 

Thus the use of small facets to create sparkles as the gem, light, or eye is moved. 

A- Reflection

A ray of light hits the diamond’s surface. 

Some of the light enters, and part of the light reflects. 

The immediate reflection is the light given off by the crown angles. 

B- Refraction

The remaining light enters the diamond and reflects toward the center of the diamond. 

The light that bounces off the internal wall of the diamond is the refraction. 

C- Dispersion 

The ray of light then shoots to the surface through the top of the diamond. 

A color spectrum is visible when light exits the diamond.

Dispersion (‘fire’) 

This involves the splitting of white light into its spectral colors as it passes through non-parallel surfaces (such as a prism).  

Light leakage 

On the flip side, wrong proportions in a cut gemstone will let too much light escape by the pavilion, creating what is called in the world of gemology/gem cutting, light leakage. 

Too Shallow

Light is lost out the bottom causing the diamond to lose brilliance. 

Too Deep

Light escapes out the sides causing the diamond to appear dark and dull. 

Some visual effects that shallow or deep cuts can create are commonly called in the industry fish eye or nailhead.  

Here are some criteria to judge the quality of a cut and polishing job on a diamond: 

Proportions 

The faceted cut is designed to create maximum brilliance and scintillation in the most symmetrically pleasing manner.

Faceted gems feature two parts, crown, and pavilion. 

The crown’s job is to catch the light and create scintillation and dispersion, while the pavilion is responsible for both brilliance and scintillation. 

Generally, when the crown height is too low, the gem lacks sparkle. 

Depth percentage

In attempting to quantify a gem’s proportions, reference is often made to depth percentage. 

This is calculated by taking the depth and dividing it by the girdle diameter (or average diameter, in the case of non-round stones). 

The acceptable range is generally 60–80%. 

Length-to-width ratio

Another measurement that is used for non-round stones is the length-to-width ratio. 

Overly narrow or wide gems of specific shapes are generally not desirable. 

Symmetry 

Symmetry is crucial for the diamonds to display all their beauty. 

Like any finely-crafted product, well-cut gems display apparent attention to detail. 

A failure to take proper care evidence itself in several ways, including the following:

– The symmetry of the sides of the table edges 

– Sharpness in the intersection of the facet edges 

– Off-center culet or keel line 

– Off-center table facet 

– Overly thick/thin girdle 

– Poor crown/pavilion alignment 

– Table not parallel to the girdle plane 

– Wavy girdle 

– Overly narrow/wide shoulders (pears and heart shapes) 

– Overly narrow/deep cleft (heart shapes)

Other bad proportions/cut problems 

Three other aspects of the cut that one should be aware of are:

1- The cullet’s size (best is no cullet) 

2- The girdle’s thickness (best is thin to medium) 

3- The girdle’s finishing (best is faceted)

In Japan, in the late 1980’s someone noticed a hearts pattern when looking at the back of a diamond through a Firescope™- the Hearts & Arrow’s viewer was born, and diamonds with H&A’s patterns became very popular in Japan. 

In the mid 90’s H&A’s arrived in the USA. 

From the top, a diamond shows an arrows pattern, and when flipped over to the pavilion side, you see eight hearts with small ‘V’ shapes. 

A true H&A’s has the patterns visible at a single glance indicating the diamond has optically perfect symmetry. 

Diamonds with H&A patterns are frequent among those with top performance. 

However, there is no proof yet that the absence of H&A optical symmetry (or any other type of pattern) means that a diamond will have lesser performance, as it’s the overall geometry of a diamond that determines its light performance. 

It is possible to produce H&A diamonds with inferior performance, and some experts suggest diamonds over 10ct should probably not be cut to show H&A’s patterns. 

H&A viewers can help to identify the level of optical symmetry in a diamond, but not its performance. 

As professionals say: “Light return is the bricks and mortar, hearts and arrows are just a coat of paint.”

Here are some examples of optical symmetry in brilliant round cuts and princess cuts: 

So, as we said, light performance is the most important. 

Professionals use a Firescope™ to see how much light a diamond returns. 

The redder the diamond saw with this instrument, the more light is returned. 

The white areas show light transmitted from behind the diamond; this is called leakage. 

The redder the diamond, the more light is returned. 

The blackness of the lens mimics an observer’s head, which blocks out some light sources.

See the chart hereunder for some examples of different diamonds observed using a Firescope™. 

Polish is graded the same way as symmetry: Excellent, Very Good, Good, Fair, or Poor on a GIA report. 

AGS adds their ‘Ideal’ Polish to the above list. 

Just as hardwood takes a better polish than softer timbers, diamonds’ hardness makes it the absolute leader in luster. 

Poorly polished facets may reduce the intensity of light reflected from or refract into and out of a diamond. 

Labs assess polish by examining the diamond, facet by facet, to see how well each of them is polished; one may not see much difference, but there is a price difference between poor polished and perfectly polished facets. 

The visual effect of Good or lower polish is that you might feel a need to clean the stone. 

During the GIA observation testing, it was found that observers were less inclined to prefer diamonds with Good and lesser polish. 

If you choose a diamond with an SI or VS inclusions, a few microscopic polish lines may be of no relevance. 

But if you were considering buying a flawless diamond, then excellent polish could be a consideration. 

If the polish is rated as Fair or Poor, visual performance may be noticeably reduced. 

The polish and symmetry grades are clearly listed in each diamond detail page and within the GIA or AGSL diamond grading report. 

For the most beautiful diamond, look for symmetry grades of excellent (EX), very good (VG), or good (G) for a GIA graded diamond, and ideal (ID), excellent (EX), very good (VG), or good (G) for an AGSL graded diamond. 

Avoid diamonds with symmetry grades of fair (F) or poor (P), as the alignment of their facets may misdirect light so severely that it affects the brilliance of the diamond.  

Diamond measurements are calculated and applied to a cut grading scale that makes it easy to understand how well each reflects light:

• Ideal or excellent cut: Represents roughly the top 3% of diamond quality based on cut. It reflects nearly all light that enters the diamond. 

• Very good cut: Represents roughly the top 15% of diamond quality based on cut. Reflects nearly as much light as the ideal cut. 

• Good cut: Represents roughly the top 25% of diamond quality based on cut. Reflects most light that enters. 

• Fair cut: Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond. 

• Poor cut: This includes all diamonds that do not meet the performance standards of a fair cut.

For the best value in a brilliant diamond, choose a diamond with a cut grade of good or very good, and polish and symmetry grades of very good or good. 

If your diamond has an ideal or very-good cut with very good or good polish and symmetry, you may want to consider less expensive grades of color and clarity — look for a diamond with G or H color and SI1 or SI2 clarity.