The cut of a diamond does not simply refer to its shape. Most gemologists consider the quality of cut, or proportions, to be the most important characteristic of a diamond. Cut determines the beauty of the finished diamond even more than color or clarity unless the diamond is of inferior quality.

The quality of cut significantly influences a diamond’s value by affecting its beauty, durability and even apparent size.

The beauty of a diamond lies in how it reflects light back to the wearer or observer. The most beautiful diamonds present the optimal balance of brilliance, fire and scintillation for their shape and cutting style.

Brilliance is the white light that is reflected from within the diamond to the wearer.

Fire or dispersion is the light refracted from within the diamond and broken into spectral colors, usually closer to the outer edges of the diamond.

Scintillation or sparkle is the light reflected off the surfaces of the diamond as it is moved.

Deviating from the “Ideal”™ or excellent standards can change the balance of these light effects.  For example, shallow angles on the crown, or top, of the diamond may slightly increase the brilliance, but the fire will be diminished. 

Symmetry and Polish: A ray of light reflects within a diamond like a house of mirrors, bouncing between facets and back to the viewer. A diamond with excellent or “Ideal”™ symmetry and polish allows each facet of the diamond to act as a perfect mirror, reflecting the light taken in from the crown area throughout the diamond and back to the viewer as brilliance and fire. Poor symmetry and polish cases the facets to act like misplaced and tarnished mirrors within the diamond, diminishing its light return and brilliance.

While diamond may be the hardest substance known to man, it is not indestructible and attributes of cut may contribute to damage during daily wear. Diamonds of any shape with an extremely thin girdle, or outer diameter, have a greater risk of chipping during normal wear. Diamonds with thin corners or points such as marquise shapes or the popular princess cuts are more easily damaged during the setting process and during ordinary wear.

A well-proportioned diamond will almost always look bigger than a poorly or commercially cut diamond of the same weight. For example, a properly cut 1 ct round brilliant cut diamond should have a diameter of approximately 6.5 mm. However, it is not uncommon to see a deep 1 ct diamond with a diameter of only 5.9 – 6.0 mm (the size of a well-cut ¾ ct round diamond). This deeper diamond will also appear darker to the eye because of brilliance lost to the observer. Why pay for weight that adds neither size nor beauty.

Why the difference in value?

The difference in value based on the quality of cut may be as much as 50% between the finest and the most commercial makes with the same clarity, color and weight. This difference is based not only in the aesthetics of beauty, but also in practical economics.

For example, a cutter buys a rough diamond crystal that could yield a 6 mm round diamond of X clarity and Y color for a set price. From that point forward, the cutter has a choice. The cutter may choose to produce a round brilliant cut of the finest proportions with comparable polish and symmetry weighing 0.75 ct. To achieve this exacting quality, the cutter may spend a significant amount of time and labor producing the diamond.

Or, depending upon his buyers, the cutter in a factory may choose to thicken the girdle and steepen the angles, cheating symmetry when necessary, to produce a poorly proportioned diamond of the same 6 mm size weighing 1.00 ct. As this cutter is also cutting other diamonds on many wheels simultaneously, his labor cost would be substantially lower.

By the time these cutters have sold their diamonds to dealers and the diamonds make their way to individual buyers, you have a very realistic scenario where an exceptionally proportioned ¾ ct diamond will have a higher value than a commercially cut 1 ct diamond of the same clarity and color.

Both the American Gem Society (AGS) and the Gemological Institute of America (GIA) labs have developed new systems for analyzing and grading cut based on the light reflected from the diamond, i.e. light performance. This is a shift away from the traditional model of cut grading using angles and proportions.

If you are interested in the theory behind the use of computer models and ray tracing technology to analyze the light performance of a diamond, please come see us in person. As gemology geeks, while we love this stuff, it gets a little lengthy for the web page.