BijanDiamonds.com Dictionary
Depth:
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 at the top of the diamond measured from one edge to the opposite edge through the middle of the diamond. Look for a diamond in which the height measures between 57.5% and 64.5% of the total diamond width in order to avoid buying a diamond that loses light out the bottom. You can usually find this measurement on the diamond certificate.