Diamond Proportions (Facets and Diamond Cut):

Pic: Screenshot of OctoNus software

Finish describes the polish of a diamond and how symmetrical, or how evenly, the facets have been cut.
Polish refers to the quality of the polish on the facets or the external clarity of the diamond. There should be no noticeable marks on the outside of a diamond. An inferior polish can be seen if a facet on the diamond has been polished against the grain. This flaw is known as graining. Noticeable graining can affect the Polish grade but has nothing to do with a diamond’s clarity. Lesser quality grades such as Good are noticeable in clarity grades of IF-VS1.
Symmetry refers to how precisely the facets are aligned to each other. It is easy to detect this property with a loupe. Poor symmetry may lead to off-center looking tables. Facets can be misaligned or not point directly to the girdle.

For more on diamond cutting see our Cut Quality section.

Girdle Thickness or height is a range of the lowest girdle or valley to highest girdle or bezel height. Girdle thickness can be very thin in the valley and very thick in the bezel. It is like waves are cut into the diamond’s side to create this effect. That is why it is expressed as a range. Each lab uses a different part of the girdle to assess the thickness; therefore it is not a very accurate measurement.

The Diameter of a diamond is the length of the girdle from one side of a diamond to another. It is possible for a diamond cutter to extract more weight out of the diamond by increasing the diameter of the diamond. For Round Brilliant cuts this would be minimum diameter - maximum diameter * depth, and fancy shapes would be length * width * depth.

A diamond cut too wide can cause light to escape through its bottom and side.

The opposite is also possible; a diamond cut too deep will allow light to escape through its side.

Taller diamonds, although they maximize weight, allow light to escape through the bottom similarly, while diamonds that are too shallow, although they appear much larger, allow light to escape out the side. The Length to Width Ratio helps determine how “well” a fancy diamond has been cut.

Total Depth or Height is how long a diamond is relative to its width.

Pic: Screenshot of OctoNus software

Table Percentage refers to the table size as a percentage of the diamond’s average width. Fancy shapes are usually characterized by larger tables.

Crown Height is the height of the girdle to the table. It can affect the dispersion of light entering and exiting a diamond. If the crown height is too small it can cause less of a fiery effect. If the Crown is not cut well it can cause light to be reflected off the diamond’s surface. Keep in mind that a mirror returns a lot of light but it’s not necessarily a good thing when your vision is obscured by one.
Crown Angle is the angle that is formed by the girdle and crown or the angle at which a diamond’s bezel facets (or, on stepped cuts, the row of concentric facets) intersect the girdle plane. This slope of the facets that surround the table is what helps to create the dispersion of light or fire, in a diamond. The crown angle also helps to enhance the brilliance of a diamond. GIA considers a well-cut cut crown to have a much smaller angle (31.8 degrees) than AGS (34.5).

Pic: Pricescope.com

Pavilion Depth is the opposite of the crown height; it is the girdle to cutlet height.
Pavilion Angle is the angle formed by the girdle and pavilion. Diamonds with large pavilion angles create a shadow called a “Nail Head.” Smaller angles produce a “Fish-Eye” effect. These are similar to the bow-tie effect in fancy shapes. These shadows were first identified by HRD and are referred to as “Kozibe.”
Star Length Percentage is the star length relative to the crown height.
Lower-Half Length Percentage is the length of the lower facets in relation to the pavilion depth.
Pavilion and Crown depth percentages should not be used as they vary between measurements of the diamond because of variances in girdle thickness.

Fluorescence is a diamond’s reaction to long wave ultraviolet radiation, especially sunlight. Some feel strong fluorescence can make a diamond less desirable while others feel that a strong blue fluorescence makes a diamond appear whiter indoors and is more desirable. Usually in diamonds F color and below as long as the fluorescence isn’t so strong that it makes a diamond appear milky, it is a good thing. If the diamond is already white as white can be, what good would fluorescence do?