Diamond Cut

 The cut of a diamond has the most impact on its overall appearance.  Rough, uncut, unpolished diamonds typically appear dull, lifeless and plain to the naked eye.  The cut of a diamond is what unleashes its radiance.  An expert cut can downplay a diamond’s imperfections while maximizing its potential. A poor cut, however, can cause even the clearest, most colorless diamond to look dark and muted. 

The brighter and more dazzling a diamond appears to be, the more light it reflects and scatters. This interplay with light is referred to as a diamond’s ‘light performance’. Light performance is evaluated by assessing the three different features: brilliance, fire, and scintillation. 

 

Understanding the basic anatomy of a diamond is essential to understanding the significance of a diamond’s cut. Take a look at Zoara's Diamond Anatomy Diagram for more detail. 

Facet: Any small, flat part of the diamond. 

Crown: The top section of the diamond

Table: Large flat facet on the face of the crown

Girdle: The “waist” or “belt” of the diamond where the crown meets the pavilion. 

Pavilion: The cone-like bottom of the diamond

Culet:  Small flat facet at the pavilion’s tip

Depth: The length, or height of a diamond measured from the table to the culet. 

Diameter: The width of a diamond measured from one side of the girdle to the other. 

To produce a diamond with excellent brilliance, fire and scintillation, a diamond cutter must fashion each diamond part to exact Proportion, perfect Symmetry, and with clean Polish. The angles, facet placement and finish of a diamond dictate its ability to return and refract light. Less light return means less sparkle. 

In a well-cut diamond, light enters, is captured, bent, and reflected back out towards the viewer, creating good brilliance, fire and scintillation. 

Light should enter the diamond, through the table.  Some of this light is reflected back immediately from the crown’s angles. 

The light that enters hits the walls of the pavilion, and is reflected toward the center of the diamond. 

Once the light is reflected toward the center of the diamond, it is then returned back through the table. 

However, if a diamond’s proportions are poor and its pavilion is too shallow, light can be lost through its bottom. 

Likewise, if the pavilion is too deep, light can be lost through the sides.  

Most gemological laboratories grade cut using the following GIA grading system. GIA uses Polish, Symmetry, and Proportion to evaluate a diamond’s light performance and to thereby asses its cut. 

Excellent: A cut that produces a diamond which reflects nearly all light that enters. Excellent cuts are very rare.

Very Good : A cut that produces a diamond which reflects almost all light that enters. Very Good cuts are rare.

Good : A cut that produces a diamond which reflects most of the light that enters. Still a good value.

Fair: A cut that produces a diamond which reflects much of the light that enters.  Noticeably less brilliant. 

Poor: Most jewelry retailers do not sell diamonds with a Poor cut grade. These diamonds usually appear dull and lifeless due to light leakage. 

AGS has a slightly different diamond cut grading system than the GIA.  ‘Polish’, ‘Symmetry’ and ‘Proportion’ are each evaluated separately with grades 0 (Ideal) to 10 (Poor).  The overall cut grade then evaluates the interplay of these three parameters in the specific diamond and the overall cut is also graded on a scale of 0 (Ideal) to 10 (Poor).  Since 2004, AGS has fused ‘Polish’ and ‘Symmetry’ into a category called ‘Finish’ and added ‘Light Performance’.  Additionally, in 2009 began offering “Platinum Light Performance Diamond Quality Documents” which include a light performance maps. Before 2005, AGS only graded round brilliant diamonds.  However, in 2005 they began to offer cut grading for princess cut diamonds and in 2006 they implemented cut grading for emerald-cut diamonds. 

In long, thin marquise, oval, pear, heart and radiant-shaped diamonds, a dark strip may sometimes appear which resembles a bowtie that can be seen with the naked eye. This dark shadow which arises from in pavilion facet alignment and angling in specifically long stones is thought to negatively impact the overall appearance of a diamond. They are also typical in especially deep or shallow stones.  The shadow is created by leaked light, or blocked light reflection. Leaked light is light that, rather than being reflected back to the viewer through the diamond’s crown, continues to travel through the bottom facets of the diamond. Blocked light reflection occurs when light is reflected back either in the wrong direction, as through the sides, or by incoming light blocked by the head of the viewer. Nearly every long, thin stone has some dark shadowing around its middle, some are less visible than others. The bow tie effect is not evaluated in diamond grading reports or general specs.  It is for the consumer to identify and evaluate for himself.