In part one of our all-inclusive guide to diamond cuts, we discussed how the Gemological Institute of America (GIA) grades diamonds. To further understand the factors determining the quality and value of a diamond cut, let us explore the proportions of a diamond, primarily its table, width, and depth, as well as its brilliance, fire, scintillation, and finishing details. Gemologists universally measure these elements, as they are excellent indicators of a diamond’s cut quality.
Facts About Diamond Proportions 
The proportions of a diamond impact its ability to reflect light and offer brilliance. The ratios between angle, size, and shape of each diamond facet are the bases of diamond cut proportions. Combinations of these elements affect how a diamond will react to light, which decides its overall beauty, appeal, and GIA grading.
About Diamond Table
Gemologists determine a diamond’s table percentage by dividing the width of the table (top surface area) by the width (diameter). For instance, if the table facet is 3.5mm wide and the diamond is 5mm wide, then the table percentage is 70%.
Light will not reflect off of a diamond’s crown angles and facets if the table percentage is too large. Vibrant reflections of color will not be visible since the light will immediately escape from the top of the diamond rather than reaching the eye.
On the other hand, if the table percentage is too low, the light will stay trapped inside the diamond and will emit through other parts of the diamond instead of to the eye.
The ideal table percentage depends on the shape of a diamond shape. Learn about 10 Best Diamond Cuts to Buy or Sell in Phoenix, AZ.
About Diamond Width
Gemologists determine the width of a diamond by measuring from one end of its girdle (diameter at its widest point) to the other end. Diamond width is the most crucial when it comes to finding out the length to width ratio, which indicates how proportionate a diamond is along with its intended shape (i.e. rectangular versus square or oval versus square).
Gemologists measure the length to width ratio of a diamond by dividing its length by its width. For example, if a diamond’s length is 5mm and its width is 3mm, then its length to width ratio is 1.67.
About Diamond Depth
Depth percentage refers to the height of a diamond, from the culet to the very top of the table. Gemologists measure the depth of a diamond in both millimeters and percentages. By dividing the depth by the width, the depth percentage is determined. For instance, if a diamond’s depth is 4mm and its width is 4.5 mm, then the depth percentage is 88.8%.
In many cases, a lower depth percentage of two diamonds with equal carats appears larger because of the increased width. On the other hand, diamonds with very low depth percentages can look darker in appearance, as they will not reflect light as effectively.
About Symmetrical Facets of Diamonds
Facets, which surround the diamond’s table, are the small mirrors reflecting light back to your eyes. Facets are found above and below the girdle. The pavilion of a diamond (the part below the girdle that reaches to the culet) also consists of facets. A round brilliant diamond, in particular, is cut with 58 facets total.
The size, symmetry, and placement of the facets play a role in how well a diamond reflects and refracts light. Any diamond with unproportioned facets, not enough facets, or too many facets can result in a less-than-ideal diamond.
About Diamond Brilliance
The brilliance of a diamond refers to the brightness of the white light reflection. When looking at this magnificent gemstone face-up under light, it should reflect an abundance of white light. When a diamond is asymmetrical, cut too deep, or cut too shallow, it will appear dull instead of brilliant.
About Fire
A diamond’s fire is the amount of colored light that reflects off of the facets and table. Well-cut diamonds not only have stunning brilliance, but fire, too. When examining a diamond face-up under light, especially daylight, you should notice colored light bouncing off of the diamond. If a diamond does not exhibit colored light reflection, it has a poor or low amount of fire.
About Scintillation
A diamond’s scintillation describes the flashes or sparkles when light moves on a diamond’s table and facets. The light and dark areas on the surface of a diamond are the cause of this beautiful scattering of light. Diamonds with a large amount of scintillation are valuable and more desirable, as diamonds without much scintillation can look dull.
Finishing Details
The finishing details refer to the craftsmanship of a diamond, including permanent treatment and polishing. The polish of a diamond helps determine the quality and condition of the facet surfaces. A well-polished diamond has a clear mirror that allows light to reflect beautifully. In contrast, a poorly polished diamond appears dull since the facets do not reflect light as vividly.
Recap: Factors for Determining Diamond Cut Quality
The cut of a diamond has always had an enormous element in determining the beauty, brilliance, and overall quality of a diamond. However, some complexities exist.
If you are looking to see how well a diamond is cut, take a closer look at how its facets and angles reflect light. To be specific, examine how sparkly and bright the light returns when under a normal lamp or in daylight.
You will also want to gauge the diamond’s brilliance (colorless light and sparkle) and fire (the rainbow light of reflection). And, be sure to keep an eye out for dark spots within the gemstone. Remember, a poorly cut diamond, even if it has a high color or clarity grade, will not reflect light as well back to your eyes, which makes it a duller and lifeless diamond.
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