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Knowing Your Way Around Round Diamonds

Round diamonds are the most common and sought-after diamond shape in the world. We stated that diamonds are polished into a variety of shapes and that the choice of shape is mostly one of taste, best use of the pre-polished rough diamond, and, occasionally, color. Historically, diamonds were polished only into rounds, and for good reason.

The science of diamond polishing was mostly developed in the past century, as light refraction became better understood. However, it was very clear to polishers from the very beginning that round diamonds had a number of advantages over other shapes they experimented with, the most important of which is light refraction. 

For a diamond to look as beautiful as possible, it must shine. The shine is light bouncing back from the depth of the diamond out of the top of it. Round diamonds return light – shine – more than any other shape diamond.

Another great advantage that round shape diamonds have has to do with inclusions. Inclusions, which detract from a diamond's beauty, are less visible in round diamonds. Thanks to these characteristics, round diamonds became the most popular shape.

The caveat is that to reach a perfect round diamond requires a lot of the rough diamonds' material; the polishing process of rounds is wasteful compared to other shapes.

Finally, an important benefit of round diamonds is that they appear larger than other shape diamonds of the same weight. This additional feature further drives the popularity of round shape diamonds.

The Perfect Shape

Diamond shapes – not to be confused with a diamond's cut – are commonly divided into three groups – rounds, semi rounds (also known as fancy shapes) and square shape diamonds. Rounds are not really a group of significantly different shapes, but variations on a theme with small differences in the number of facets and where they are located. The variations are attempts to improve a little light reflection as well as to create brands that are differentiated from other rounds.

In all cases, the issues of value are similar: the size of the diamond, its quality and color and its cut grade.

When discussing shapes in general, we presented a table that showed the difference in value between the various shapes. In the table, rounds had the highest value at 100% and the values of all other shapes were percentages of the value of rounds. An important reason for that is that rounds have the highest cost per carat. Cost, in this context, is the expenditure on rough, which should not to be confused with value.

It's All in the Yield

Rounds, just like other diamond shapes, are polished from rough diamonds that have the most fitting model to begin with. A fitting model is one that results in the least amount of material loss during the polishing process. Generally, from a manufacturing standpoint, rounds are the most wasteful in terms of yield and are therefore generally the most costly diamond shape in terms of weight. Depending on the shape (model) of the rough diamond, during the polishing process of round cuts, as little as around 45% and as much as around 72% of the material is lost.

As part of the Mercury Crystal Clear framework, we described in previous articles how different characteristics impact a diamond's value. This time we will examine a characteristic of diamonds that final-buyers rarely see – the rough diamond model – and how it relates to cost.

The first example (​All Image courtesy OGI) shows a 24.15-carat rough diamond that will result in a 9.20 carat polished round diamond. As the image on the left in the 3D imaging below clearly shows, the pre-polished planning makes full use of the diameter of the rough diamond. Despite this attempt to maximize yield, the side view on the right shows that much of the rough diamond will be lost. In this case, the yield is just 38.1%, meaning that 61.9% of the rough diamond is lost.

When One Becomes Two

Sometimes, a rough diamond presents a polisher with several options. The next 3D image shows a rough diamond weighing 11.91 carats that can yield a 4.61 carat polished diamond. The resulting polished would be an F color, VS2 clarity, Excellent Make, round diamond. 

 The above example results in a very nice polished diamond. However, in the polishing process, the yield would be just 38.7%. That is slightly better than in the first example, but with a rough diamond of this model there is another option: polishing two diamonds out of it.

In the following 3D imaging, we see that the same rough can be sawed and then polished into two round diamonds. The color and clarity of the resulting polished are unchanged – F, VS2 – but with this approach the manufacturer gets a 0.78-carat diamond in addition to a larger 4.49 carat diamond.

Extracting two diamonds out of a single rough instead of just one is preferred in this instance because it requires only a very small compromise in the weight of the main diamond and the manufacturer gains an extra polished diamond. In terms of yield, the outcome is better. In the above example, it increases to 44.3%. More than half of the rough is lost, but there is an improved return on the cost of the rough diamond.

This situation includes a tradeoff – to get a second diamond the polisher needs to compromise on the size of the first, larger diamond. In this case, the polisher will consider the value of the first option resulting in a single large diamond versus the total value of the two smaller diamonds in the second option.

In the case above, where all polished diamonds had the same color and clarity, and the larger diamond in the second option is very close in weight to the single diamond in the first option, the choice is clear: the manufacturer will have a better return on the cost of the rough diamond by pursuing option two.

A Matter of Value

Another comparison is that of value. Much effort is required to bring out the value of the round diamond. This is because reaching a round diamond requires so much of the rough diamonds' material. Furthermore, polishing a perfect round diamond with proportions resulting in an Excellent cut grade may result in an even lower yield (depending on the rough diamond shape and any inclusions).

Below is a value comparison between a round shape diamond with the best polishing (triple excellent), a round diamond with less than perfect polishing (good, good, good) and a perfectly polished pear shape diamond. It is an analysis of current market prices of 2-carat diamonds.

As stated in previous articles, the intention of this review is to highlight a number of issues relating to diamonds, their value and how to understand the differing values of diamonds that may seem identical but differ enough to impact their value. These concepts are important to keep in mind when considering diamonds as part of a wealth preservation belief or even as a gift. 

This comparison demonstrates the importance of light return. A triple good round shape diamond returns less light than a triple excellent, and the value of the diamond reflects that – a 28% loss of value. Although fancy shapes (in this example a pear shape) generally have lower values than rounds, with a top make their light return is high and therefore their value is higher than a triple G round diamond. This is commonly true for all fancy shapes. Why this is not necessarily true for square shapes will be explained in an future​ article.

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