A gemologist grading a diamond always measures the proportions it possesses. There are five main ratios accepted commonly within the diamond industry: table percentage, depth percentage, pavilion angle, and crown angle. These help in identifying cut grade of a stone, and are as such included on the grading report accompanying it.
This ratio is among the key factors that decide cut grade, and is numerically equal to the distance from table to culet, over the diameter of the stone across its girdle. Depth should generally be between 54% and 66% of the diameter if the sparkle is to be optimal. Diamonds with Very Good and Ideal cut grades possess a depth percentage inside this range.
This ratio is between the table width of the stone and its girdle diameter. A stone with too much or too little width in relation to the diameter would have a flat or rounded top respectively; either way, it would leave something to be desired in the sparkle. The ideal range is between 53% and 70%.
Too large a crown angle, and the stone’s sparkle can become limited when one looks at it from the top. Small crown angle leave the stone looking the glassy and clear, as opposed to filled with a dazzling shine. An optimal angle ensures you get the best sparkle out of the thing. Round diamonds mostly possess a crown angle in the range of crown angle 30% to 35%. For an Ideal cut diamond, this should measure 33.7% to 35.8%.
Just like the crown angle, the pavilion angle too needs to be just right for a stone to look its possible best. Too large, and it blocks a significant portion of the sparkle; too shallow, and the diamond ends up looking glassy. When cutting a stone for brilliance, it should be noted that the pavilion is responsible for most of the sparkle, which comes out the top of the stone. The best pavilion angle falls between 42.5% and 43.5%.
How Diamond Proportions Work
- Diamonds that are too shallow: The depth percentage is too shallow, and table percentage is too low. The pavilion and crown angles are too shallow.
- Diamonds that are too deep: The depth percentage is too deep, and table percentage is too high. The pavilion and crown angles are too large.
- Correct diamond proportions: The depth percentage and table percentage are both ideal. The pavilion and crown angles are optimal.