In an ideally-proportioned diamond, all of the light entering the diamond from the top will bounce within the diamond and be reflected back through the top, giving the stone maximum brilliance and fire. If the stone is too shallow or too deep, some light will escape through the bottom part of the diamond, giving the appearance of shadows when viewed from the top.
It's easy to see that the deep-cut diamond shown above will have a higher carat weight, but is clearly the less desirable stone! Many jewelers will not discuss cut proportions unless the customer specifically asks; a stone rich in carat weight but poorly proportioned can be deeply "discounted," giving the buyer a false impression of a great deal.
Common Proportion Metrics
In order to assess how well a given diamond is cut to ideal proportions, you will have to measure the diamond. If the stone has a G.I.A. certificate, the measurements will be on the certificate. Otherwise, ask the jeweler to perform the measurements in front of you.
There are five common metrics used to evaluate the proportions of a diamond. How they are calculated and what they mean are summarized in this table:
Metric How to Calculate Impact on Proportion Quality Depth percentage c ÷ a × 100 (%) This is the most important metric for maximum brilliance. Diamonds that have too small or too large depth percentages will be too deep or too shallow, and will have shadows when observed from the top. Table percentage b ÷ a × 100 (%) The bigger the table percentage, the greater the brilliance and the lesser the fire, and vice versa. The ideal table percentage strikes a perfect balance. Crown height percentage d ÷ a × 100 (%) Deviation from the ideal will cause reduced brilliance and fire. Pavilion depth percentage e ÷ a × 100 (%) If significantly greater than the ideal, the diamond will have a dark center. If significantly less, the stone will look watery and lifeless. Girdle thickness Qualitatively look at f Numbers are not used to measure the girdle thickness -- the girdle is judged from extremely thin to extremely thick. Thin girdles tend to chip; thick girdles "hide" extra, unnecessary weight. An even, medium girdle is ideal. Crown angle g (degrees) Another way to measure Crown height percentage Pavilion angle h (degrees) Another way to measure Pavilion depth percentage Length/Width Ratio (a' ÷ a) : 1 By definition a' is the longest side, and a is the shortest. For round and square stones, the length/width ratio should be close to 1. Other shapes have specific ratios that are most desirable.
The ideal proportion metrics vary between different shapes (rounds, hearts, ovals, etc.) Continue to the next page to understand the ideal proportion metrics that will yield the most brilliant diamond for the shape of your choice.