Diamond Cut Part 1: Table and Depth Percentage

The cut of a diamond determines how well the stone reflects light. This reflection is responsible for the gem’s brilliance or overall sparkle. There are many aspects to a stone which contribute to angles and the cut form, as the diagram to the left indicates.

Diameter: The width of the diamond through the girdle.
Table: The largest facet at the top.
Crown: The Top part from the girdle to the table.
Girdle: The narrow band around the widest part.
Pavilion: The bottom part from the girdle to the cutlet.
Culet: The facet at the bottom tip. The preferred cutlet is not visible to the naked eye.
Depth: The height from the cutlet to the table.

Professional and skilled cutters cut the stone to mathematically precise proportions to allow as much light as possible to reflect from each of its mirror-like facets out of the top of the diamond.

The diameter and depth of the gem affect how light travels within it and how light transgresses in the form of brilliance.

If the cut is too shallow, light escapes out from the sides and the diamond loses brilliance.  If the cut is too deep, light is lost from the bottom and the diamond appears dull or dark.

Without attention to quality cutting, light is lost and not returned to the eye. When looking for a great cut, the two most crucially important numbers to pay attention to are the depth percentage and the table percentage.

The depth percentage is the depth of the diamond (from table to culet) divided by the width of the diamond. The higher the number, the deeper the stone. The lower the number the shallower the stone.

The table percentage is the length of the table divided by the width of the diamond. The higher the number, the bigger the table looks. The lower the number, the smaller the table looks.

The proportions displayed by the diamond are very significant.  The two key factors in the grading of cut quality – table and depth percentage – are normally recorded on any decent grading report.   Three different parameters are measured to allow for easy calculation of these percentages by using the formulas expressed below.

For example, for a round brilliant cut diamond, table percentage is calculated as follows:

And depth percentage is calculated as follows:

Although every cut represents the opinion of its designer, there are certain proportions which best balance the qualities of brilliance or ‘sparkle’ and dispersion or ‘fire’ within a round diamond.  These are indicated by the proportions perfected by certain designers show below.

For more information on diamond cut, please keep an eye out for part two of this series where we explain Crown and Pavilion angles!