Diamond Grading and Measurements

As we discussed earlier, the diamond’s cut is the most important grading element to determine beauty and value. You can have a flawless, colorless diamond and if it is not cut correctly the stone will not be pretty or give much fire or brilliance. You can also have a SI1 clarity diamond with J color, cut to correct proportions, and you will have a brilliant diamond in spite of the less than perfect grades in other categories. This is why cut is the first grade you should consider when buying or grading a diamond.

Every facet and angle of a diamond is placed there by a cutter who controls the final product. Each cutter evaluates every diamond to achieve the best light return for the stone, as well as to get the best value return for their investment in the rough. They have to weigh out various factors when cutting the stone. As a result, some diamonds are cut better than others, since the better cuts can cause a greater weight loss. Therefore, when grading a diamond or making a buying decision, you have to make these same determinations: mainly to consider what is the quality of the cut of the diamond, and how that quality relates to the final value of the diamond.

In the previous sections we looked at some of the finer points of symmetry and some errors found in cut evaluation. These errors are considered minor as long as they do not adversely affect the beauty of the diamond.

Below, however, we will look at the evaluation of diamond cutting as applied to the beauty of the stone; at how you determine the numbers that go into that evaluation; and at some practice stones to see what a well cut diamond...and a poorly cut diamond...looks like. First, let's see what the optimum numbers are. This is the mathematical equation that affects every facet and angle you see in a diamond. A diamond that has been cut with the best combination of these numbers is called an Ideal Cut, a term established by the American Gem Society. This organization is the diamond industry frontrunner for cut evaluation and the only organization to issue a cut grade for diamonds.

Below we see the various numbers that deem a diamond to be an IDEAL CUT diamond. Notice that even the Ideal Cut has variations. For instance, the table percentage can range from 53% to 57%. This is because these numbers may vary slightly and still give optimum light return or brilliance. Later we will see how you obtain these diamonds in a cut grade procedure of diamond grading.
 

Ideal Cut Diamond Proportions

The first question that most people ask is "How do you know if its a well cut diamond?" Which is a good question. There are many instruments on the market that will automatically grade the cut of a diamond. These are expensive and sometimes difficult to obtain for a gemologist or jeweler...and certainly for a consumer. But there are a few simple measurements that you can understand and know to look for that will help you get a practical determination of a diamond proportioning and cut grading.

First you need to know the width of a diamond. And this is usually put forth as two numbers: the highest and the lowest. To get these you simply measure a diamond in several directions until you get the widest and narrowest dimension. And these can vary very slightly or a lot. But you will never find a diamond that is a perfect circle. The numbers will always vary.

Measuring a diamond for width	Take measurements in many directions to get the greatest and least dimensions. Then take your average to use as your cut proportioning number. Once you have the greatest and least, you add them together and divide by two to get your average diameter of your diamond. This number becomes the control number, which is used to obtain all other measurements and divided by to get you various percentages.
Overall depth of the stone	Depth is obtained by measuring the diamond from the table to the culet. Now that you have the depth, all you need to do is divide the depth by the control number from your diameter and you will get your overall depth percentage. A range in the 59% to 62% is the most desirable. More or less and you may have light leakage due to the stone being too deep or to thin as shown in the demonstration images earlier in this lesson.
Diamond total depth examples: Left is 57.4% (Too Shallaw) Right is 61.1% (Just Right)
Table percentage	To get this you will need to measure the table from corner to corner...not side to side. Now that you have your table measurement, you can divide that number by the control number from your width. This will give you your table percentage which will also be a very important cut evaluation factor. Here are some example of the various table sizes and how they affect the diamond.
Diamond table size percentage examples
Relation of table to crown height	You will also find that there is a relation of the table size to the crown height. A larger table will usually result in a flat crown, while a small table will result in a higher crown. The importance of this is that a smaller table will usually result in the diamond has more dispersion or fire. This is the light being broken up into its spectral colors like a prism. But with a larger table, the light return will be more brilliant, that is to say more white light will be returned without being broken up in the spectral colors. This is why some people prefer a larger or smaller table. And should be a consideration to anyone thinking of buying a diamond.

Final Thoughts on Diamond Cut Grading

For most consumers, the GIA Grading Report will be the diamond-grading certificate most often seen. The GIA grading report does not offer a cut grade but does provide the numbers for the cutting factors that we have seen so far. This includes depth, table, girdle, culet and finish as polish/symmetry. So that if you see these reports or an AGS Diamond Quality Document, you should be able to identify the various factors being discussed and make a good decision based on the numbers provided. You can learn how to be able to evaluate a diamond's cut grade for yourself.

As I stated before, there are a lot of organizations doing a lot of research on the grading of diamond proportions as related to the return of light by the stone to the wearer. And there are not any hard and fast rules about whose research is most correct. But I urge consumers to not get caught up in all of the hotly debated cut grading issues regarding who is right and who is wrong. You should choose a diamond that is pretty to you and that the diamond sparkles for you. Because you may be told which diamond is the best based on numbers on a page, but that diamond may not be the one you like best. So be aware of the numbers, know what they mean, and you should be able to pick out the best diamond for you.