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The brilliance of a stone is in part a function of how it is cut. Most gemologists consider cut the most important diamond characteristic. Light entering the gem bounces around on the inside and is reflected off of the pavilian. A diamond for example, which is perfectly cut has a proportion which will maximize light being aimed back up through the top. If a stone is cut too deep or too shallow, it will not reflect as well, lose light through the bottom, and have a duller appearance. Ideally cut stones have more fire and scintillation. Generally, the crown should be a third of the depth of the pavilion. Diamond cuts are also graded on other characteristics such as:

 

Fire: Like a prism, the flashes of colored light reflected back from within a diamond is called its fire. The higher the grade diamond, the more refracted colors when the gem is turned in the light.

Scinillation: The flashes of light seen when a diamond moves. May also describe the patterns of dark and light areas seen in a gemstone.

Depth Percentage example: Let's say you have a stone which is 7mm in diameter, and 4.1mm deep. The formula would be: 4.1 / 7 = .586 or 58.6%.

Table Percentage example: Let's say the same stone which is 7mm in width has a table which is 4.2 mm wide: 4.2 / 7 = .60 or 60%.
There is no industry-wide standard for what exactly is best, or what the categories are. However, as an example the one on the right from BlueNile.com details how Depth and Table Percentages affect Cut Grade in a round cut diamond. The diamond in question would be graded as "very good" as far as these two criteria are concerned.

Cut Grade Depth % Table %
Signature Ideal 60.1 - 61.9 55 - 57
Ideal 60.1 - 62.8 53 - 57
Very Good 58.5 - 64 52 - 60
Good 57.5 - 64.5 51 - 65
Fair 56.5 - 67 50 - 67
Poor <56.5 or >67 <50 or >67
Girdle Width: The girdle is part of a jewel normally grasped by prongs when it is mounted into a setting. The thickness of the girdle may range from being too thin to too thick. Too thin and it will be prone to chipping. Too thick and it takes up pavilian space used to refract light.

Symmetry: The uniformity of a gemstones cut is described as its symmetry. Common problems with symmetry in jewels are facet evenness, misalignment of facets, extra facets, symmetry of shape, slanted tables, misshapen tables, off-center tables, off-center culets, damaged culets, crowns that are misaligned with the pavilian, and out-of-round girdles. All of these factors can effect the brilliance of the gem and reduce its grade and value. The GIA (Gemological Institute of America) does rate symmetry and there are seven ratings from Excellent to Extremely Poor. When purchasing a stone, you should be fine with a symmetry rating ranging from Excellent to Good.



Sources and Resources
About Cutting and Polishing - Tradeshop.com
Gemstone Cuts - BlueNile.com

Gemstone Cuts - Wikipedia.org
Gemstone Flaws - NitaCreations.com
Jewelry Lab Certificates and Appraisals - Novori.com
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