Advanced Diamond Tutorial Part 4

Diamond Cut: The Basics – Cut, Part 1 – Facet Structure
Chapter 4
Having a well-grounded understanding of a diamond’s facet structure is critical to gaining a full understanding of cut. In this chapter, we are going to breakdown the facet structure of a round diamond and discuss the basic purposes of the facets of the diamond. In the following chapters, we will be taking each shape, with its unique facet structure and discussing it, its cut, what measurements, angle combinations, and facet lengths bring out the stone’s beauty.
The Table Facet
Figure 4.1
Graph Showing Diamond Table Facet

The table facet is the largest facet on the diamond. It is common to all main-stream shapes (Round, Square, Cushion, Pear, Marquise, etc.), and cut patterns produced in the market. The major function of the table facet is to allow light to enter the diamond through the top. Since a diamond’s sparkle is actually light that is being reflected up out of the diamond, it is critical that large amounts of light be able to ender directly into the diamond. In order to allow a maximum amount of light to enter the stone directly, a large, flat facet on the top of the diamond is essential.
The “perfect” table size has been a topic of fierce discussion and debate between experts over the years, with each side being deeply entrenched in their own opinions. As with many areas of diamond cut, many of the experts’ positions overlap, showing that there is a certain amount of veracity and consistence to each of their arguments. Rather than to take a stance with any one school of thought, we prefer to look at the intersection of the major, scientifically supported theories.
There is no “one size fits all” when it comes to table measurements. Each one is unique in the way that it interacts with the other measurements around it, and certainly, from shape to shape, proper table measurements take on totally new ranges and rules.
We will be looking closely at optimal ranges for table measurements a little later on when we break down our discussion of cut for each different major diamond shape on the market.
Something that is very important to remember, not just with the table facet but with all facets on a diamond, is that they live in a 3 dimensional world and can move in multiple directions. 
Most often, when a facet’s measurement is discussed, we talk about its diameter, depth, length, or angle. However, a facet can “move” in other ways that need to be considered. For example, is the table off-center? Is the table tilted, in relation to the girdle of the stone? Is the table warped, or skewed? Is the shape of the table symmetrical? Is the facet pattern of the stone symmetrical? Are the facets on the pavilion and crown lined up with each other? Is the culet off-center? Is a facet shifted, twisted, rotated, beveled, curved, etc. While this is a topic that we will not be touching on again until later, it seemed fitting to mention it here in order to set the tone for discussions to come. 
The Star Facets
The star facets are small, triangular facets that surround the table, in stones that exhibit the “brilliant cut” pattern, such as rounds, ovals, pears, hearts, marquise, princess, radiant, etc; as opposed to a step cut pattern, such as an emerald or asscher; or even a proprietary cut, such as a Lucida, Criss-cut, etc (we will address their facet structures later on). See figure 4.2.
Figure 4.2
Graph Showing Diamond Star Facets
The stars are the first of three sets of facets that make up the angled portion of the diamond’s crown. Light that enters these facets will be bent as it passes through them, in contrast to light that enters through the table. See figure 4.3 and 4.4. (These illustrations are of a diamond cut to AGS Ideal proportions, using a single “ray-trace light source” in the model to illustrate the path that light takes through the stone.
Figure 4.3
Graph showing diamond light reflection
Figure 4.4
Graph showing diamond light refraction
The facets that make up the crown of the diamond are very important, because they will bend light passing through the diamond in two way, when it enters the stone, and when it exits the stone. If these facets are cut to angles that are inconsistent or improper, it will have significant effects on the sparkle and brilliance of the diamond.
The most common measurement used to express the dimensions of a star facet is that of length, which is expressed in the percentage of the distance which the facet covers between the edge of the table and the edge of the girdle (this was already discussed in the first chapter of this tutorial).
Again, there are many schools of thought as to what is the optimal measurement. Rather than proclaim the perfect measurement, it is more informative to look at how different star facet lengths affect the diamond, and how their measurements, when combined with various measurements from the pavilion of the diamond, will yield different optical results, within in which it is possible to find differing appearances which may appeal more to one person than another. The result of such truths is that no one measurement is “the best”, but rather a range of measurements and combinations should be considered. We will breakdown the different measurements and combinations for each shape later on in this tutorial.
The Bezel Facets
The bezel facets are the major facets in the crown of a diamond with the “brilliant cut pattern”. It is the angle of these facets that is depicted as the “crown angle” on the lab reports. These are the largest facets in the crown, and have the greatest impact on the light performance of the stone. The bezel facets of a diamond are illustrated in Figure 4.5, shown below.
Figure 4.5
Graph showing diamond bezel facets
The important statistic to consider about bezel facets is their angle. Since they are the major facets of the crown, they impact the bending of light more than the other two facet groups on the crown.
The key to finding a diamond with a good or great cut is not in the in angle measurement of the bezel facets alone, but rather in the combination created by the angle of the bezel facets and the pavilion main facets (pavilion main facets will be discussed later in this chapter). It is this combination that will produce the bending and reflecting of light. If only one set of these facets fall into the proper range, the variance in the non-conforming set of facets will offset the other.
Focusing on good combinations and how this will affect the diamond’s ability to reflect light is key if you want to find a diamond that is going to have the “wow” factor.
The Upper Girdle Facets
The upper girdle facets are those facets that have one edge touching the girdle. In stones with a “brilliant cut pattern” especially rounds, these small facets can play a very big roll. See Figure 4.6.
Figure 4.6
Graph showing diamond upper-girdle facets
There are two issues that can happen with these facets that can affect the appearance and beauty of a diamond. They will both be covered under the section discussing the cut of a round diamond. So you know to look out for these topics, they are called Painting and Digging. As with any other portion of diamond cut, these points are hot points of controversy and argument. We will thoroughly discuss these issues a little later.
The Pavilion-Main Facets
The pavilion-main facets, sometimes called “the mains”, are the major facets that extend from the culet of the diamond to the edge of the girdle. These facets are responsible for the majority of the light reflection from the pavilion of the diamond. It is the angles of these facets that will determine where the light that enters the diamond will be reflected.
Figure 4.7
Graph showing diamond pavilion main facets
The measurement used to define this facet set is an angle measurement. You can see this measurement represented on a lab grading report as the “pavilion angle”. This measurement is a very important one, and, particularly in rounds, a very small change in the angle can result in huge impacts to the visual beauty and sparkle of the diamond.
Again…as with the crown angle…there is no one measurement that is the “best”. It is all about the combination of the crown and pavilion angles, working together to produce beautiful optics. Of course, there are limits, and angle measurements within certain ranges that tend to produce the nicest diamonds.
As we move through the coming chapters on cut, we will examine different shapes, and how changing the angles of the pavilion-main facets can affect the performance and beauty of a diamond.
The Lower Girdle Facets
The lower girdle facets, as their name would suggest, and just like the upper girdle facets, are the facets that extend from the edge of the girdle, down towards the pavilion. They are found in between the pavilion main facets and are grouped two together.
Figure 4.8
Graph showing diamond lower-girdle facets
The primary measurement used to describe the lower girdle facets is that of length, expressed in a percentage…which is the percentage of the distance the facets cover between the edge of the girdle and the culet.
Their length is significant, as it can affect the visual beauty and appearance of a diamond, although their effect is not as great as the “pavilion mains”. Also…like their opposites (the upper girdle facets), lower girdle facets can also suffer the affects of Painting and Digging…which we will discuss later.
The Culet
The culet is the point at the bottom of the pavilion. The purpose of the culet is really to keep the diamond from being damaged on the bottom. Today, it is most common to see culets that are referred to a “None” or “Pointed”, since most diamond are brought to a sharp point at the bottom. This was not always the case however. Particularly in more antique cuts, it is quite common to see culets that are flattened, creating another facet on the bottom of the diamond.
The issue caused by having a large girdle is that it creates a facet that is parallel to the table. This opens an exit point for light to pass through at the bottom of the diamond, which allows light to pass directly through the diamond, without being reflected back to the viewer, i.e. “light leakage”.  Although the look of a large culet can be very enticing, especially in an antique stone, it is important to understand that it will allow a fair amount of light to be lost through the bottom of the stone, and will affect the overall light return of the diamond.
It is important to remember, once again, that diamonds are personal, and that something that one customer may consider undesirable, could be sought after and prized by another.
The Girdle
The girdle is the thin “band” around the widest part of the stone. The girdle is very important to the cut of a diamond, but not so much in the way of light performance. Rather, the girdle has to do with the structural soundness of the diamond. After all…the diamond will be held in the setting by the girdle, and, while being worn on the finger by its owner…there is a greater chance that the wearer will bang or knock the stone on its girdle than any other part of the diamond.
The girdle measurement, on a lab grading report, is expressed in either millimeters, or as a percentage of the stone’s total depth. It is important to have a girdle that is the right thickness. The girdle should be thick enough to be secure for setting, and secure against chipping or cracking, however if the girdle is too thick, this will hide “extra weight” in the stone. “Extra weight” is carat weight that you pay for in the price, but don’t see in the actual millimeter diameter of the diamond.