The cut of a diamond refers to the way in which it is angled and proportioned.
Better Cut = More Expensive
The cut is arguably the most important of the 4Cs because the intensity of the sparkle and brilliance of the diamond is mostly due to the quality of the cut. The different angles and lengths contribute to how much of the light that enters the diamond is reflected internally and then back out again. This is the diamond’s “sparkle”.
It is also one of the most difficult of the Cs to rate. One particular cut may appeal to one person but not to another. Before discussing the specifics of the cut it would help if you were familiar with the names of the major measurements of a diamond. The figure below provides a good illustration of these.
Above Figure: Major Diamond Parameters
When you visit a jeweller/merchant they will use these names when describing a particular diamond's proportions to you.
When a diamond is graded by an independent laboratory all of the proportions are taken into account to arrive at a grading of the proportions (cut). The diamond grading certificate will state what standard the proportions (cut) are. For example the GIA grading will state that the proportions are either:
There are precise mathematical formulas for determining the cut of a diamond that gives the optimum amount of sparkle.
One of these types of cut is called the “Ideal Cut”. The figure to the left shows the path a ray of light takes when entering an Ideal Cut diamond. The light is reflected back up to the viewer of the diamond, and appears to have more sparkle.
Above Figure: Light Path Through Ideal Cut Diamond
The figure below shows an example of the proportions that make up a typical Ideal Cut. There are a range of proportions that exist, not just one value.
Above Figure: Ideal Cut Diamond Parameters
With regards to the shape of the diamond, the round cut gives the most intense sparkle because it suffers from the least amount of "leakage”. Leakage is the amount of light that is reflected out of the sides of the diamond rather than the top.
The figures below illustrate the concept of leakage of light. On the left is a shallow cut diamond and on the right is a deep cut diamond.
Figure: Shallow Cut Diamond Light Path Figure: Deep Cut Diamond Light Path
Each of the flat surfaces of the diamond is called a facet. The figure below illustrates the various names of the facets of a diamond.
Figure: Diamond Facet Names
A diamond’s facets will either let light pass through or reflect the light. The angle that the light hits the facet determines whether the light is reflected or passes through.
The girdle is the circumference of the diamond that marks out its diameter. The girdle should be an even depth right around the entire diamond.
Figure: Three Examples Of Girdle Thickness
Ideally the girdle should be somewhere between thin-medium to slightly thick. If the girdle is too thin then it may be prone to chipping. If the girdle is too thick then the diamond carries its weight around its middle rather than being a wider diameter and you get a diamond that looks smaller than what it should for the price you pay.
The culet is the point of the diamond.
Assessing The Quality Of The Cut
Initially, assessing the quality of the cut will be difficult for you to do. Cut is one of the most subjective of the 4Cs to assess because one individual will assess sparkle intensity differently to another.
As you have done with the other 3 Cs, calibrate your Cut radar by asking your jeweller to compare an Ideal Cut diamond with a poorly cut diamond. You should easily be able to notice the more intense sparkle that the Ideal Cut exhibits.
Then, have your jeweller bring out progressively better cuts of diamond to compare to the Ideal Cut diamond, and note the point where you can no longer make a distinction between the difference in sparkle.
A second element of the diamond cut is the shape of the diamond. The most common of these for an engagement ring is the round shape.
Some shapes popularity increases as trends change. If you are looking for the most sparkle from your diamond then stick with the round shape diamond as the geometry of it means that there is less leakage compared to the other types of diamond shape.
HCA – Scientifically Evaluating a Round Diamond’s Sparkle
An Australian master jeweller by the name of Garry Holloway devised the Holloway Cut Adviser (HCA) as a scientific means of determining the amount of sparkle that a round diamond exhibits, based on the proportions of that diamond.
Holloway’s test uses four factors to describe the quality of the diamond’s visual appearance. These are brilliance (light return), fire, scintillation and spread. These are described below.
Brilliance (light return)
Brilliance is the perception of a diamond’s brightness. A bright diamond will return a lot of light back up to an observer.
Fire is a measure of the dispersion of the light that is returned back to a diamond’s observer. A diamond with good fire appears to have a lot of flashes of rainbow colours. This aspect of the diamond’s appearance is best viewed with less light, otherwise the fire flashes get swamped by the sparkle of the diamond.
Scintillation is the intense sparkles that occur when a diamond is moved about. Ideally there are many areas of scintillation across the face of a diamond with few dull patches.
Spread is a measure of the apparent size of a diamond and should be an important factor in your buying decision. Obviously you want to be able to purchase the largest diamond you can with the money you have available, while also factoring in quality for the other 3 Cs. Whilst size doesn’t play a large part in the beauty of a diamond, it certainly contributes to the desirability of that diamond.
The depth percentage value is used to give a range for a good diamond spread. Using Figure 8, if you divide the depth of the diamond by the diameter of the diamond you should get a value that is somewhere between 56% and 65%.
As with the other Cs, proof of a diamonds’ cut can only be verified with an authentic laboratory certificate.
The laboratory certificate for the diamond will give values for proportions of the cut such as:
Shape e.g. brilliant
Measurements, i.e. diameter and depth
Assessment of proportions, e.g. very good
Depth or % of girdle
Type of culet (the point of the diamond)
Some of these proportions can be used in a calculation that determines the values for the:
Tips on Diamond Cut
When evaluating a diamond’s cut:
Use the laboratory certificate measurements to determine that the diamond has an optimum spread of between 56% to 65%.
Start with an ideal cut and a poor cut so you are able to gauge the standard of a diamond’s sparkle. Then get progressively better diamond cuts to compare against the Ideal Cut until you can’t tell the difference.
Make sure the girdle isn’t too thick as the diamond will be hiding its weight in the girdle area and won’t look as big. Conversely the girdle shouldn’t be too thin as it will be more prone to chipping.