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Diamond Grading

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Diamond Grading And Measurements


As we discussed earlier, a 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 produce 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

Diamond MeasurementsEvery 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. When buying a diamond mainly consider: What is the diamond's cut quality? Also, How will the quality of the cut affect 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.

In the diagram, however, we will look at the evaluation of diamond cutting as applied to the beauty of the stone. How you determine the numbers that go into that evaluation. We will see some practice stones to see what a well cut diamond and a poorly cut diamond look 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.

Above we see the various numbers that deem a diamond to be an "Ideal Cut". Notice that even an ideal cut diamond 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, fire or brilliance. Later we will see how you obtain these diamonds in a cut grade procedure of diamond grading.

What Is A Well Cut Diamond?


The first question that most people ask is "How do you know what a well cut diamond looks like?" 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 evaluation of a diamond's proportions 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. These can vary 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


Diamond Reflecting LightTake 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


Diamond Reflecting LightDepth 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 ImagesTable Percentage


Diamond Reflecting LightTo 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 ImagesRelation Of Table To Crown Height


Diamond Images
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 having more dispersion or fire. The term fire means the light being broken up into it's 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 into the spectral colors. This is why some people prefer a larger or smaller table. You should take this into consideration when 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 we stated before, there are a lot of organizations doing research on the grading of diamond proportions as it relates to the return of light in relation to the wearer. There isn't any hard and fast rule about whose research is the most accurate. We urge consumers to consider the research simply to make an informed decision. You should choose a diamond that is pretty to you and which sparkles the best. Even if your told a diamond is the best for you because of it's measurements, still that diamond may not be the one you like best. So be aware of the numbers and know what they mean, and you should be able to pick out the best diamond that suites you.

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