The most important factor when considering a diamond is the quality of its cut.
When we talk about cut, we are talking about the precision with which the diamond is cut and polished. There are half a dozen well known shapes into which diamonds are usually cut. Each shape affects the overall look of the stone. But it's how well each shape is executed according to the science of light performance that is assessed when a stone is graded.
Diamonds cause light to pass through them in a way that allows for an incredible display of light called brilliance and fire. But a poorly cut diamond does not live up to the expectation we have of diamonds.
A great deal of research has gone into determining what proportions make for maximum brilliance.
These standards are used by the labs that grade the diamond. If a diamond's facets, cut angles, and ratios, fall within the parameters that have been determined as "Excellent" this is noted on a grading report. In order to make this process as uncomplicated as possible there are only very few cut grades used by GIA. The cut grade appears as either: Excellent, Very Good, Good, Fair, or Poor. This is the range of cut grades provided by GIA for standard round brilliant cut diamonds in the D-to-Z color range.
It's generally felt by most diamond lovers that anything less than an Excellent cut grade is a lost opportunity and best avoided.
How GIA grades a diamond's cut grade
While there may be a great deal of science involved and fussing over number and angles it all comes down to how the diamond serves the buyer. A grade of "Excellent" is given to diamonds that look the best while not being too delicate. GIA takes into account how well the diamond interacts with light, but they also make sure that the girdle is not too thin nor too thick. They measure he stone's brightness or how white light reflects from the diamond. GIA also notes the diamond's fire which is diamond-speak for "the scattering of white light in all the colors of the rainbow". GIA will also take into consideration the stone's scintillation or what can be thought of as "the pattern of light and dark areas caused by reflections within the diamond".
How well the diamond is designed, faceted, and crafted influences the above factors. So part of the analysis includes an examination of the diamond’s weight relative to its diameter, the diamond’s girdle thickness which affects its durability, the symmetry of the diamond’s facet arrangement, including the angles and relative positions of all the facets, and the quality of the polish on the facets.
These factors are all assessed to give the diamond its overall cut grade.
While polish and symmetry are mentioned separately on a diamond's grading report, a diamond will not receive an Excellent cut grade if there are serious deficiencies in its polish or symmetry.
In one sense, diamonds are just little rocks. But they are extraordinary rocks, jam-packed with value and significance. Clarity, Cut, and Culture (Susan Falls, 2014)
A hundred diamonds of the same size can each have a different price, and the variations in price can even be quite extreme. This is because diamonds come in different levels of quality. In order for a diamond cutter to cut a diamond perfectly (remember they are small for what is being done to them) a huge amount of skill is required. The skill involves not only the actual cutting and polishing, but also the visualizing of the diamond before the first cut is ever made. Yes, there is some fancy tech involved, but the attention to detail by the diamond cutter is crucial. There are some pretty fine details involved. This is what makes a cut diamond a work of art. The final value bestowed on it is not only the result of how well the diamond was cut. It is also determined by the quality of the rough.
The term “diamond cut" means several things. Diamond cut in fact has two distinct meanings: cut quality and cut shape. This is what confuses many people. But soon they come to understand that cut is used to not only refer to the shape but it also is used when talking about a diamond’s beauty. The diamond’s beauty is a result of how it returns light, how brilliant it is, how much fire and scintillation it produces and how much inner contrast it exhibits. In technical terms the cut quality grade refers to a diamond’s facet proportions and angles. Cut quality is important to a diamond buyer because of the 4Cs it provides the best value in terms of how much beauty is produced for the amount of money that has to be spent. Size (carat weight) increases correspond to a much greater increase in price than a better cut does. The same goes for improved clarity. Flawless, and Internally Flawless stones are prohibitively expensive and provide comparatively little advantage for the increase in cost. An improved cut however, though it may cost a little more, does not result in as steep an increase in the price of the diamond when considering the benefit it provides. Diamonds are all about attractiveness. A super ideal cut diamond is something that “Wows”, while a poorly cut diamond looks plain in comparison. Ideal and super ideal cut diamonds are not the majority. Most diamonds don’t have a great quality cut. To understand a little more about how a cut quality affects a diamond’s performance let’s go over how a diamond differs from other clear materials like glass.
Light can pass through a diamond but it will also reflect off its inner and outer surfaces. A diamond cutter’s goal is therefore to angle the facets in such a way so that the majority of the light entering the stone is reflected off the inner surfaces and returned to the eye. This light is called scintillation. At the same time they will take care that the facets direct some of light in such a way as to break them up into the full spectrum of colors that make up visible light. These colorful flashes of light are called “fire”. There’s always a delicate balance between scintillation and fire since the light is being directed outward through the crown in two different forms: white light and broken up light. As a general rule the stones which have a marginally deeper cut will give more fire, while a stone which is slightly shallower will have greater brilliance.
It is not possible to cut every diamond to ideal proportions which maximize light return. The reason being that diamonds have inclusions. Some inclusions are unsightly and need to be cut away. But since diamond is very valuable a diamond cutter will try to cut away as little as possible. If they aren’t careful they might cut away all their profits. Too much cutting and they will find the diamond sells for less than they bought it for. So they have to figure out some way to cut the diamond even if it doesn’t end up ideal. It stands to reason that many diamonds have what are called “poor” proportions. These will be referred to having a poor “make”. The truth is that many people coming in to buy a diamond only think about how much the diamond weighs. They want a 1 carat diamond or a 1.5 carat diamond, and aren’t informed enough about diamonds to know that a 1.5 carat diamond can look smaller than a 1.4 carat diamond if it has a deep cut. The lack of sparkle will also make the diamond look smaller next to one with a lot of sparkle. They also might not be aware that a poorly cut diamond will look darker and will therefore require one to look among higher and more expensive color grades in order to find the highly coveted colorless stones. So you can see how why the cut plays such an important role in the quality of a diamond.