The cut of a diamond is graded according to its geometric proportions. Many gemologists consider cut the most important characteristic because even if a diamond has perfect color and clarity, a poor cut will dull a diamond's sparkle. The considerations below will help you determine your preferred grade of cut for a round diamond.
From the rough crystal, many diamonds are cut to maximize carat weight at the expense of brilliance, but some are cut to excellent proportions. In a well-cut diamond, almost all the light that enters is reflected out of the top and sides in the form of fire, or colored light, and brilliance, or white light. The figure below illustrates the paths of light in differently cut round diamonds. If a round diamond is cut too deep or too shallow, light is lost out the sides and bottom. Because good-, very good-, and ideal-cut diamonds reflect the most light.
An ideal cut produces the maximum amount of reflected light. Because the round diamond is the only shape that has complete symmetry - required for 100% light reflection - only round diamonds may be graded as ideal cuts.
A very good cut reflects back nearly all the light that enters the diamond. With the unaided eye, you cannot see the difference between a very good cut and an ideal cut round diamond, and even the differences in measurement are minute. Because other diamond shapes do not have 100% symmetry, very good is the highest cut grade to be given to any fancy shape diamond, like oval, emerald, or princess. A very good cut diamond is considered an exceptionally proportioned, heirloom-quality gemstone.
A good cut describes a well-proportioned diamond that reflects a good percentage of light. A diamond with a good cut is an above-average cut gemstone, making it very appropriate as a jewelry gemstone.
A fair cut maximizes diamond weight at the expense of fire and brilliance. A fair cut might reflect as little as 40 percent of the captured light.
A poor cut results in a lifeless-looking diamond. Poor-cut gemstones are not recommended for fine jewelry.
This chart below will you determine the quality of the cut. The table percentage is the diameter of the top facet (or table) compared to the diamond's width. For a diamond with the most sparkle and fire, we recommend a diamond that has cut proportions that fall within the Good range to Ideal. These measurements are stated on the diamond certificate.
Below is an illustration defining the various parts of a round diamond.
Shapes other than the standard round brilliant are called fancy shapes or fancy cuts. Their names are based on their shapes. The best known are the heart, marquise, pear-shaped cut, emerald-cut, oval, and radiant. Please see the illustration below.
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