The following diagram is a very “well cut” diamond and it’s proportions. On diamond grading reports you will see percentages for the table, crown, pavilion, and total depth.
These are percentages in relation to the 100% figure, which is the diameter of the diamond (average diameter for round brilliant cuts and the width for fancy shape diamonds). Also listed are 2 important angles, the crown angle (34.5°) and the pavilion (40.75°)
> Table – The largest facet on a diamond. Mostly responsible for the amount of white light returning out of the top of the diamond. The quality and quantity of this white light coming up through the table is also called the brightness or brilliance of a diamond.
The crown dispersion(the angled facets between the table and the girdle) is mainly responsible for the breaking up of the white light (that is returning up from the pavilion) into its spectral colors. This is called fire or dispersion. Just like you would see as white light passes through a prism (in the picture).
As you make the table larger it will reduce the crown area and therefore it will increase the amount of “brightness” and decrease the amount of “fire” in that diamond.
It can be a matter of preference… I like tables to be less than 56% (giving it a larger crown area, so a bit more “fire”) and some people like the look when the table is 60% or higher (giving it a smaller crown area, so a bit more “brightness”).
> Crown – The crown is an angle and a percentage. The crown facet angle should be set to about 34.5° degrees…. if this angle starts to become shallower than 32° degrees there is a decrease in fire and an increased chance of durability problems.
The crown height percentage is the area from the top of the girdle to the table. A 53% table with 34.5° degree crown will give you a 16.2% crown height. As the table percentage becomes larger or the crown angle becomes shallower, the crown height percentage will become less.
> Girdle – The girdle should be thick enough to protect the diamond, period! If it is any thicker than that, it is just adding extra weight and if it is thinner it can present durability problems. Normally a girdle of a Round brilliant Cut diamond that is in the range of Thin to Medium to Slightly Thick is acceptable. Sometimes it may be polished or faceted.
> Pavilion – As with the crown, the pavilion has an angle and is also a percentage. The pavilion angle is very critical!…. if there is a variation from 40.75° degrees it will drastically affect the light performance of the diamond. The pavilion must act like a mirror. The pavilion height percentage is the area from the bottom of the girdle to the culet.
> Total Depth Percentage – The measurement from the top to the bottom of the diamond (depth) as compared to the average diameter. For example…. 4.04mm depth divided by 6.50mm average diameter= 0.6215 times 100 = 62.2% total depth percentage.
As shown above, the total depth percentage can be deceptive because it is the grand total of the crown height percentage, the girdle thickness, and the pavilion depth percentage. Two diamonds could have exactly the same total depth percentage but the three separate sections of each diamond could be very different.