The depth of a diamond is its height (in millimeters) measured from the culet to the table. On a grading report, there are normally two measurements of depth – the first is the actual depth measurement in millimeters (shown under ‘measurements’ at the top of a grading report), and the second is the depth percentage (see image on left), which shows how deep the diamond is in relation to its width.
Today, the art of polishing diamonds had become somewhat scientific. That said, even though the depth percentage can be a tell tale sign for brilliance and value (stones that are cut too deeply are usually cut to retain weight from the rough), it doesn’t tell the entire story. You will find out why as I explain things in more details.
The place where that depth lies is also crucial in determining the diamond’s beauty. The pavilion, in particular, should be cut at correct proportions so that light rays can bounce around within the diamond and be reflected out at the proper angle to meet an observer’s eye.
Total depth percentages of 56.6% (left) and 65.9 (right) are recipes for disastrous looks.
The ideal depth percentage varies with the shape of the diamond. A depth percentage that may be too much for one shape might be essential for another. For instance, a princess cut with a 75 or 77 percent depth would still be considered acceptable and can yield an attractive diamond. On the other hand, a depth of 65 percent for a round diamond would be excessive and be detrimental to its beauty.
The table refers to the flat facet of the diamond which can be seen when the stone is face up. It also happens to be the largest facet on a diamond and plays a vital role on brilliance and light performance of a stone.
The main purpose of the table facet is to refract light rays entering the diamond and to allow reflected light rays from the pavilion facets back into the observer’s eye.
It is widely misconceived that a larger table percentage would make a round diamond more brilliant than one with a smaller table. However, this is not the case as there are other factors that contribute to the overall brilliance and fire of a diamond.
Fire is best observed at the bezel facets of a diamond. With a finite amount of space at the crown area, having a larger table would mean that bezel and upper girdle facets would now have less surface area and dispersion decreases. Vice versa, having a small table would allow girdle facets to disperse light more effectively while suffering from issues with brilliance.
It is therefore important to strike a balance between light transmission through the table and color dispersion through the crown and upper girdle facets. As they say, too much of a good thing may do more harm than good.
An insanely huge table of 81% will make the stone on the left devoid of dispersion properties.
In a grading report, table percentage is calculated based on the size of the table divided by the average girdle diameter of the diamond. So, a 60 percent table means that the table is 60 percent wide as the diamond’s outline.
If a diamond is round, gemologists can compute this value by dividing the table diameter, measured in millimeters by the average girdle diameter. For a consumer, this can easily be calculated based on the average measurements found at a grading report’s top left-hand side.
For fancy shaped diamonds, table percentage is computed by dividing the table width (measured at the widest part of the facet) by the width of the widest part of the stone measured in millimeters. On a grading report, this width is the second of the three values under “Measurements” and is delineated in millimeters.
Combined, the depth and table percentages of a diamond play a major role on the stone’s beauty. It shows you how the stone has been cut in relation to its proportion and is usually one of the first few values to help you instantly weed out diamonds with less ideal cuts.
While the variations in brilliance and sparkle may be apparent to the eye, the subtleties of proportioning may not be easy to discern. Here’s the limitations imposed by numbers on a piece of paper. Even if a diamond has values that lie within “ideal” ranges, the diamond still requires additional performance data to determine its cut precision. We’ll show you how to select a diamond correctly in this guide here.
Also, while a diamond’s spread will be affected by its table width, choosing the best personality is a matter of personal taste ultimately.