In several previous articles of the GIA On Diamond Cut series, we have emphasized the importance of considering every proportion when examining the overall appearance of a round brilliant cut (RBC) diamond. This is true whether we are creating computer models of light behavior, or trying to predict the visual appearance of an RBC diamond with certain proportions. We believe that the traditional approach—considering only the table size, crown angles, and pavilion angles (or table size, crown height, and pavilion depth)—needs to be updated to include other proportions that can significantly alter the appearance of a diamond and the amount of light it will return.

In a series of articles on optics (Diamond Optics: Parts 1, 2, and 3) and the article on The Importance of Three-Dimensionality, we discussed the relationships between these proportion parameters and light return. In this article we will show how changes in each of seven different parameters change the appearance of the diamond's shape. Although the effects of changes in the traditional proportion aspects are familiar to those who regularly work with diamonds, changes in the non-traditional proportion parameters (e.g., star facet length and lower-girdle facet length) produce some surprising results. In addition, we have included an animation that demonstrates why the reliance on a particular total depth (here, 60%) might sometimes be misleading.

Below you will find a link to several animations and a short description of which proportion is being changed. Each link shows an animation in a new pop-up window. Diamond images for most animations are shown in both profile and face-up (or pavilion-up) views. In each animation, one proportion is varied while the rest of the proportions remain fixed.

The fixed proportions have the following reference values:

• Crown angle = 34°
• Pavilion angle = 40.5°
• Table size = 56%
• Star facet length
  (percentage of total distance from table to girdle) = 50%
• Lower-girdle facet length length
  (percentage of total distance from girdle to culet) = 75%
• Girdle thickness (at the thickest point) = 3%
• Culet size = 0.5%

The GIA Diamond Proportion Animations

Table Size: The table size in this animation changes from 50% to 70%. There is a significant difference in both the face-up and profile views. Crown height and total depth are affected by this proportion change.

Crown Angle  : The crown angle in this animation changes from 25° to 43°. Notice that there is a substantial difference in the profile view, but hardly any change in the face-up view. Once again, crown height and total depth are affected by changes in this proportion feature.

Star Facet Length: The star facet length in this animation changes from 40% to 70%. Although the differences in appearance are more pronounced with the face-up view, the profile appearance of the crown also changes quite a bit. However, the crown angle and crown height are not affected by changes in this proportion aspect. The impact of variations in star facet length usually has been omitted from evaluations of the quality of an RBC diamond's cut.

Girdle Thickness: The girdle thickness (measured at its widest part) changes from 1.5% to 13% in this animation. With increasing girdle thickness there is an increase in the total depth of the diamond, which can add significant "unseen" weight to the diamond when viewed face up.

Pavilion Angle: As the pavilion angle in this animation changes from 39° to 44°, there is a distinct difference in the profile view, but hardly any change in the pavilion-up view. Both pavilion depth and total depth are affected by changes in this proportion parameter.

Lower-Girdle Facet Length: The lower-girdle facet length in this animation changes from 50% to 95%. Both the profile and pavilion-up views show a significant change in appearance. However, the pavilion angle and pavilion depth are not affected by this change in proportion value. This is another proportion aspect that usually has been omitted from evaluations of the quality of an RBC diamond's cut.

Culet Size: The culet size in this animation changes from 0% (sometimes called a pointed culet, or no culet) to 4%. Although the change in culet size does not seem that large in comparison to the overall diamond, it produces a noticeable difference in the appearance of the pavilion in both profile and pavilion-up views. The pavilion depth and total depth are slightly affected by this proportion change.

60% Total Depth: This animation keeps the total depth of the diamond constant at 60% while alternately changing the table size and crown angle. First, the table size is varied from 53% to 64%. Then the crown angle is varied from 25° to 41°. For each of these changes, the pavilion angle (and therefore the pavilion depth) must be adjusted to keep the diamond's total depth at 60%. These animations show that a wide range of profile appearances is possible, including some that are quite unusual.

We hope that you found this article useful, and invite any feedback or comments that you may have. You may contact us by email at DiamondCut@gia.edu.

All diamond images were created by T. Scott Hemphill.
Diamond Module and Flash animations were created by Barak Green.

The GIA Diamond Proportion Module is
© 2002 Gemological Institute of America, Inc.

[Footnote 1]  All download times are determined by assuming optimal bandwidth connection, and may be affected by changes in connection speeds. [back]