BijanDiamonds.com Dictionary
Depth Percentage:
The depth percentage is the height of the diamond (measured from the tip of the culet to the table) compared to the diameter of the diamond (the top of the diamond measured through the middle from one edge of the girdle to the other). To avoid buying a diamond that loses light out the bottom, look for a diamond in which the height measures between 57.5% and 64.5% of the total diamond width. You'll find this measurement on the diamond certificate.