Every diamond is unique. However, all diamonds share certain structural features. Diamond anatomy determines its proportions, brilliance, dispersion and scintillation. Having a basic understanding of how each part contributes to the diamond will help you find your perfect stone.
Diamond Table Size
The table is the top horizontal facet of the diamond which is also the largest facet, regardless of the diamond’s shape. The average table size is expressed as a percentage of the diamond’s average girdle diameter.
Table Size % = Table Average Diameter / Girdle Average Diameter x 100
If the width of the table is too large or too small compared to the diamond’s average girdle diameter, the top of the diamond will appear too flat or rounded respectively. Either way, any disproportion affects a diamond’s brilliance and sparkle which diminishes the stone’s beauty.
Here is the list of the ideal table percentage ranges for the most popular diamond shapes: round – 53 – 58%, oval – 53 – 63%, pear – 53 – 63%, marquise – 53 – 63%, princess – 67 -72%, cushion – 61 – 67%, emerald – 61 – 69%, Asscher – 61 – 69%, radiant – 61 – 69%, heart – 53 – 63%.
Diamond Total Depth
Total depth or height is measured from the surface of the table to the culet, in other words from top to bottom. This measurement is expressed as a percentage of the average girdle diameter.
Total Depth % = Total Depth (mm) / Average Girdle Diameter x 100
Total depth percentage is one of the main factors when determining a diamond’s cut grade. When a diamond has the right combination of total depth and girdle diameter, it is more capable of reflecting light which results in a stronger and more beautiful sparkle.
Here is the list of the ideal total depth percentage ranges for the most popular diamond shapes: round – 59 – 62.3%, oval – 57.5 – 62.0%, pear – 58 – 62%, marquise – 58 – 62%, princess – 64 – 75%, cushion – 61 – 67%, emerald – 61 – 67%, Asscher – 61 – 67%, radiant – 61 – 67%, heart – 58 – 62%.
Diamond Pavilion Depth
The pavilion is the lower portion of a diamond from the bottom edge of the girdle to the culet. It is expressed as a percentage of the average girdle diameter.
Pavilion Depth % = Pavilion Depth (mm) / Average Girdle Diameter x 100
A diamond with a too deep pavilion is not capable to reflect the entering light properly as the light leaks away from the first pavilion facet. On the contrary, the light entering a diamond with a too shallow pavilion reflects off of the opposite crown facet instead of bouncing across a second pavilion facet.
An ideal diamond should have such a pavilion depth percentage when the entering light doesn’t leak away from the opposite pavilion facet but is returned to the observer through the table.
Diamond Pavilion Angle
Diamond pavilion angle is the dimension that highly affects a stone’s brilliance. This is the average of the angles formed by the diamond’s pavilion main facets and its girdle plain.
If the pavilion angle is too large, the diamond will not emit notable sparkle while if it is too shallow, the gem will appear glassy.
According to Marcel Tolkowsky, the optimal pavilion angle degree is 40.75. However, unless you are looking for a super-ideal diamond cut with a pavilion angle of 40.9 degrees, opting for stone within a range of 40.6 – 41 degrees is quite safe.
Diamond Crown Height
The crown is the upper portion of a diamond, from the top edge of the girdle to the table. Crown height is expressed as a percentage of the average girdle diameter.
Crown Height % = Crown Height (mm) / Average Girdle Diameter x 100
This dimension affects both dispersion (fire) and brightness of a diamond as crown and table facets act as windows driving light return.
Diamond Crown Angle
Crown angle is the angle formed where the bezel facets meet the girdle plane. This parameter has a large effect on the face-up appearance of a diamond.
Surprisingly the crown angle can compensate for the leaking light by the first pavilion facet, meaning if the pavilion angle is steep, the crown angle has to be shallow to maximise the light return and vice versa.
According to Marcel Tolkowsky, 40.75-degree pavilion angle is perfectly paired with a 34.5-degree crown angle. The recommended crown angle ranges from 34 to 35 degrees.
The girdle is the middle portion of a diamond, narrow section separating the crown from the pavilion. It functions as a diamond’s setting edge. Girdle thickness is described as a range from its thinnest to the thickest parts.
Girdle Thickness % = Girdle Thickness (mm) / Girdle Average Diameter x 100
The Gemological Institute of America (GIA) grades the girdle using an eight-grade scale: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick and Extremely Thick.
A thick girdle is not desirable as it unnecessarily adds weight to the stone where it matters the least. For example, a diamond of 2 carats with thick girdle will appear smaller than a diamond of the same weight and thin girdle.
From the other side, an extremely thin girdle is more fragile and exposed to chipping. Therefore, a girdle in the range of Thin to Slightly Thick is preferable.
The culet is a small facet at the bottom of a diamond placed parallel to the table. It prevents chipping and abrasion to the point. Size of the culet can affect face-up appearance of the diamond.
Culet Size % = Culet Size (mm) / Average Girdle Diameter x 100
The culet is described according to its size using the following grades: None, Very Small, Small, Medium, Slightly Large, Large, Very Large and Extremely Large. When there is no culet, it may be indicated as a pointed culet.
Preferably the culet should not be visible with a naked eye which is why None, Very Small or Small culet falls in the “Excellent” range.