Judging Cut Quality
When discussing the cut of a diamond, the term cut to jewelers usually refers to the shape, the cutting style, the proportions of a stone, and the finish of the stone (such as the polish and the symmetry).
Proportions of a diamond include such features such as how big or small the table facet is, and how deep or shallow the pavilion is. The diamond proportions and finish are also called the make of the stone.
With regards to the polish, this includes such features such as the smoothness of the diamond's surface, polishing marks, rough girdle, scratches, pits and abrasions.
Symmetry refers to any misaligned facets, culets or tables that are off-center, out-of-round girdle outlines, wavy girdle, misshapen or symmetrical facets. It's important to note that no diamond is 100% symmetrical. Diamonds that appear lopsided or have irregular facets are not acceptable. Avoid buying diamonds that have these features.
Cut Grade Factors
|Total Depth Percentage=||Depth of the Diamond|
The total depth percentage should be about 60% which is a good ratio for a round diamond.
|Crown Height Percentage=||Crown Height|
The crown height percentage is not entirely necessary to calculate. It's easier to judge the diamond's crown by looking at it. It will be obvious if the crown is too low or too high.
Crown Angle- If a diamond has a crown angle that is higher than 40 degrees will have a high crown. If a diamond has a crown angle that is less than 30 degrees it will have a thin crown. According to the American Gem Society, a good crown angle for a round brilliant cut diamond is between 33.7 and 35.8 degrees.
Pavilion Depth Percentage- If the pavilion depth percentage is 40% or less, the diamond will have a fisheye. When the pavilion depth is 49% or more, the stone will have a dark center. According to the American Gem Society, the ideal depth percentage for the pavilion is between 42.2% and 43.8% for a standard round brilliant cut diamond. This percentage is the most important proportion factor because it relates to the brilliance of the diamond.
Cut Grading Terminology
Brightness: is the actual and/or perceived amount of light that is returned by the diamond.
Dispersion: the separation of white light into rainbow colors.
Pattern: is the size and arrangement of bright and dark areas on a faceted diamond.
Leakage: areas that do not return light in the diamond.
Contrast: is the degree of difference between bright and dark areas. A diamond with a high contrast is desirable.
Painting and Digging Effects: is a technique used to alter the angles of the upper and lower girdle facets as well as to remove blemishes at the girdle. This can sometimes cause uneven scalloping around the diamond's girdle.
Weight Ratio: is the weight of the diamond in relation to its diameter.
Overweight Percentage: is the degree to which the diamond is heavier than a reference diamond with standard proportions. To calculate this percentage, you would have to take the size difference of your particular diamond (from the standard) and divide it by the standard size then multiply the result by 100 to get the overweight percentage. The higher the overweight percentage, the less valuable is your diamond.
Durability: refers to a diamond's resistance to chipping or breaking. Diamonds containing extremely thin girdles and crowns can chip easily.
Tilt: is the point at which the girdle reflects under the table facet when tilted. If there is a see-through effect when the diamond is tilted, then the diamond is not well-cut.
Star Facet Length Percentage: is the percentage of the total distance between the girdle edge and the table edge. According to the GIA, an excellent star facet length is between 45% and 65%.
Lower Girdle Facet Percentage: is the percentage of the total distance between the girdle and the culet. According to the GIA, an excellent lower girdle facet range is between 70% and 85%.
Judging a Diamond's Brilliance
A diamond that is extremely brilliant is well-proportioned. In order to examine the brilliance of a diamond, the stone should be cleaned and viewed face-up. Brilliance is described as a mirror-like quality resulting from the reflection of light off of the surface and the interior of the stone. The more light that returns, the more brilliant the diamond.
It's important to view diamonds with the naked eye as well as with a hand magnifier. You should examine the diamond under different types of light such as fluorescent tubes, halogen spotlights, and daylight. When judging a diamond, try to observe the diamond at a reasonable distance from the light source. If a diamond is too close to the light source, the diamond will appear more brilliant than it actually is.
When analyzing the brilliance of a diamond, it's best to compare it to a reference standard (i.e., a brilliant diamond that's well-cut) and to have the assistance of a competent salesperson.
When judging diamonds with the naked eye you should be able to see brilliance throughout the stone. There shouldn't be any dark or washed-out areas which allow you to see through the bottom of the stone.
Poorly Cut Diamonds
Nailhead: is a diamond with a dark center. This is a result of a pavilion that is cut too deep.
Bow Tie: a fancy cut diamond that has a dark bow tie shape within the diamond due to the pavilion's facets not being cut properly. Most fancy shaped diamonds exhibit this but it's slightly noticeable.
Fisheye: a white circle located within the center of the diamond. This is caused by the reflection of the girdle due to a shallow pavilion. The thicker and more prominent the white circle is, the poorer the cut of the diamond.
The proportions of a brilliant cut can affect the way that light is returned out of the top of a diamond. Diamonds that are poorly cut will have dull brilliance.
(a) An ideal-cut diamond has the ideal proportions for a brilliant cut diamond. It reflects nearly all of the light that enters it. The crown should be one-third of the depth of the pavilion in order to be considered an ideal-cut.
(b) If a stone is cut too deep, the light is reflected through the sides. The stone will have black spots and an uneven appearance.
(c) If the pavilion is too shallow, light will leak through the bottom of the stone. It will have a dull reflection and a glassy "window" effect.
Ideal Cut-Nearly all the light that enters the stone is reflected. This is the highest grade for a diamond.
Very Good Cut- Reflects almost the same amount of light as an ideal cut but is less expensive.
Good Cut- Reflects most of the light that enters the stone and is less expensive than a very good cut.
Fair Cut- Not as brilliant as a good cut.
Poor Cut- Diamonds that are too deep or too shallow that light leaks out of the sides and bottom.