The total depth percentage is a very important element of a diamond's proportions and it is greatly responsible for the amount of brilliance that a diamond will display. Diamonds which have ideal to very fine cut with proper total depth percentage provide the greatest brilliance.
Ideal to Very Fine Cut Diamonds have total depth percentages between 59% and 63%. Diamonds within this range usually provide excellent brilliance. Diamonds that have depths outside these ranges usually cost less, but a degree of brilliance is lost. When depth percentage becomes greater than 65%, the center of the diamond becomes dark as light refracts through the bottom of the diamond.
However, proper total depth percentage does not guarantee proper cut. Total depth percentage is very important but it can sometimes be misleading. Total depth percentage is a representation of a diamond's overall total depth. It is possible for two diamonds to have the same depth percentage and be proportioned differently.
Total depth percentage is a combination of many other important elements of diamond cut. These elements include crown height, crown angle, pavilion depth percentage, pavilion angle and girdle thickness. These elements are properly proportioned in ideal cut diamonds.
However, some diamonds with total depth percentages that are in the range of ideal may be improperly cut and display less brilliance. A diamond with ideal total depth percentage may also have an overly deep bottom, (deep pavilion depth percentage & steep pavilion angles) and have a very low top, (low crown height & shallow crown angle.) A simpler example would be two men that are six feet tall. One man has perfect symmetry. His upper body is in proper proportion to his lower body. The other man has very long legs and a short waist.