Science of BeautySymmetry

Proportions

So what are the proportions that make a really good looking diamond? As noted before, there is no one size fits all, however, there are general guidelines that are likely going to make a stunning stone.

The first thing you should look for is the official grade given by a respectable laboratory, such as GIA or AGS. GIA will assign round and princess cut diamonds a grade out of 5: Excellent, Very Good, Good, Fair and Poor. AGS will give a grade out of 11, with 0 being the best cut and 10 — the worst. The second way to judge a diamond is by its proportions. They are measured as percentages of the total diameter (or width for non-round shapes) of the diamond. The best proportions for a round brilliant diamond will lie in the ranges listed below.

When a particular measurement is just out of the range above, a good looking diamond is still quite possible, but the light performance will not be optimal. Very often though, a diamond may be cut for weight rather than beauty. The pavilion may be cut too deep, resulting in a diamond that looks too small for its weight and exhibits less brilliance. These diamonds should be avoided, since they are made only to be sold to unsuspecting buyers who think they are getting a large stone, when in fact, that stone would need to be recut and lose about one fifth of its weight in order to really look beautiful. At the same time, some diamonds are cut with very shallow pavilions, resulting in a stone that may look big, but lacks sparkle. When a variation of any dimension from the guides above are too great, a stone's brilliance will suffer. Such cutting decisions arise from working with rough diamonds of unusual shapes and a desire to keep as much of carat weight as possible. In reality this is nothing more than dead weight which would need to be cut off in order to bring the diamond to its true potential.

1: Light refracts inside a well cut diamond and escapes through the table, resulting in bright appearence. 2 and 3: Light does not refract properly inside a diamond that is cut with too shallow or too deep a pavilion and escapes before reaching the table, resulting in dark appearence.

The table diameter is a measurement that, other things being equal, does not change the quality of the cut in itself, but rather its character. A diamond with a large table will have a greater brightness and scintillation, but less dispersion, since dispersion only occurs with light rays travelling through the crown facets. A smaller table yields a diamond with lower overall brightness, but with far more colourful sparkles. A smaller table will also make the diamond appear smaller and results in higher weight loss during cutting for most common shapes of the rough diamond. This is part of the reason why fancy cuts like emerald, radiant, asscher and cushion are marketed: they look best with a larger table (around 70%) and allow for better utilization of the rough.

Notice that the table above includes crown height and pavilion depth, but not total depth. While it is more common to talk about total depth, it alone cannot tell everything about the stone, since it does not indicate what exactly are the ratios of crown and pavilion depth, and it is those ratios which define the actual path the light takes inside the stone. Total depth also includes depth of the girdle, which has little to no effect on the stone's beauty, but does have an effect on weight, and therefore price.

When choosing a diamond with a table size that is different from recommended, pay attention to the crown height values. Since refraction of light depends on the angle at which it strikes the surface, most cutters, when forced to enlarge the table will try to keep the angle at which the crown meets the girdle constant at around 34.5°. This means that the crown height will need to decrease. When presented with a diamond with a table diameter of 70%, you can be sure it will not look awful if the crown height is about 10%, which would be too low for a stone with a table of 57% — arguably the best table size for a round diamond.

As noted before, these proportions do not apply to shapes other than round brilliant. Shapes such as marquise and heart are especially difficult to standardize to specific proportions. When buying fancy shaped diamonds in person it is best to simply look carefully and see which diamonds attract you the most. When buying online and unable to see the diamonds 'in action', it is safe to assume that all other things being equal, a more expensive diamond will have better proportions. Still photographs that are sometimes included with diamond descriptions online, although worth a thousand words, are not complete enough to tell you how the diamond will look live, though it can be useful for judging symmetry. As a general guideline, the table diameter and depth of the fancy cut diamonds can often be much higher than for round ones and produce very well cut stones.

Interestingly, larger diamonds are cut for beauty more often than smaller ones. This is due to two interrelated factors: firstly, more expensive large diamonds are much more likely to be bought by a public that can tell a difference between a well cut stone and a poorly cut one and appreciates superior cuts, and secondly, the price for a slightly smaller well cut stone is on average higher than for a slightly larger poorly cut one. Applying logic to these factors it becomes clear that the second factor is a direct consequence of the first one. This is a simple law of supply and demand at work.

Science of BeautySymmetry