Diamond Cut - Cut Grading

Cut Grading

The "Cut" of the "4-Cs" is the most difficult part for a consumer to judge when selecting a good diamond. This is because some certificates will not show the important measurements influencing cut (such as the pavilion angle and crown angle) and will not provide a subjective ranking of how good the cut was. The other 3-Cs can be ranked simply by the rating in each category. It requires a trained eye to judge the quality of a diamond cut, and the task is complicated by the fact that different standards are used in different countries (see proportions of the round brilliant).

Other proportions also affect the look of the diamond:

• The table ratio is very significant.
• The length of the lower girdle facets affects whether Hearts and arrows can be seen in the stone, under certain viewers.
  • Most round brilliant diamonds have roughly the same girdle thickness at all 16 "thick parts".
  • So-called "cheated" girdles have thicker girdles where the main facets touch the girdle than where adjacent upper girdle facets touch the girdle. These stones weigh more (for a given diameter, average girdle thickness, crown angle, pavilion angle, and table ratio), and have worse optical performance (their upper girdle facets appear dark in some lighting conditions).
  • So-called "painted" girdles have thinner girdles where the main facets touch the girdle than where adjacent upper girdle facets touch the girdle. These stones have less light leakage at the edge of the stone (for a given crown angle, pavilion angle, and table ratio), but does have a negative effect on the overall beauty of a diamond.

Several groups have developed diamond cut grading standards. They all disagree somewhat on which proportions make the best cut. There are certain proportions that are considered best by two or more groups however.

• The AGA standards may be the strictest at the upper range of quality. David Atlas developed the AGA standards in the 1990s for all standard diamond shapes.
• The HCA changed several times between 2001 and 2004. As of 2004, an HCA score below two represented an excellent cut. The HCA distinguishes between brilliant, Tolkowsky, and fiery cuts.
• The AGS standards changed in 2005 to better match Tolkowsky's model and Octonus' ray tracing results. The 2005 AGS standards penalize stones with "cheated" girdles. They grade from 0 to 10.
• The GIA began grading cut on every grading report for round brilliant beginning January 1, 2006 based on their comprehensive study of 20,000 proportions with 70,000 observations of 2,000 diamonds. The single descriptive words are as follows: Excellent, Very Good, Good, Fair, and Poor.

The distance from the viewer's eye to the diamond is important. The 2005 AGS cut standards are based on a distance of 25 centimeters (about 10 inches). The 2004 HCA cut standards are based on a distance of 40 centimeters (about 16 inches).

• Various labs around the world are using ImaGem's VeriGem device to measure Light Behavior. DGLA in the USA and Mumbai, India, PCGL in the USA and EGL-USA are both offering versions of this grading in 2008. DGLA has graded thousands of diamonds with this promising direct assessment technology.
• "Brilliancesope" by Gemex is another assessment light behavior technology in use by many US and now foreign retailers and diamond cutters.