Brilliant (diamond cut): Difference between revisions

The relationship between the crown angle and the pavilion angle has the greatest effect on the look of the diamond. A slightly steep pavilion angle can sometimes be complemented by a shallower crown angle, and ''vice versa''.<ref>{{Cite book|url=http://www.folds.net/diamond/index.html|title=Diamond Design - A Study of the Reflection and Refraction of Light in a Diamond|first=Marcel|last=Tolkowsky|editor-first=Jasper|editor-last=Paulsen|publisher=London: E. & F. N. Spon, Ltd., New York: Spon & Chamberlain|year=1919|orig-date=Web edition 15 November 2003|archive-date=2023-03-12|archive-url=https://web.archive.org/web/20230312204834/http://www.folds.net/diamond/index.html|url-status=live}}</ref>

** 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, i.e., (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 girdleit. These stones (such as EightStar-brand diamonds) have less light leakage at the edge of the stone (for a given crown angle, pavilion angle, and table ratio). Some diamonds with painted girdles receive lower grades in the GIA's cut grading system, for reasons given in a 2005 GIA article.<ref name="paint"/>

Several groups have developed diamond cut grading standards. TheyThese allstandards disagree differ 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. David Atlas, (who developed the AGA standards), has suggested that they are overly strict.

* The American Gem Society (AGS) standards changed in 2005 toin order better to match Tolkowsky's model and Octonus' ray tracing results.<ref>{{cite web | url=http://www.octonus.com/oct/projects/ | title=OctoNus Software, Moscow State University, Bruce Harding, and others have posted work there. | access-date=2013-01-28 | archive-date=2013-01-23 | archive-url=https://web.archive.org/web/20130123194608/http://www.octonus.com/oct/projects/ | url-status=live }}</ref> The 2005 AGS standards penalize stones with "cheated" girdles. They grade from 0 to 10.

*The GIA began grading cut on every grading report beginning in 2006, based on their comprehensive study of 20,000 proportions with 70,000 observations of 2,000 diamonds.<ref name="paint">{{cite journal|last1=Blodgett|first1=Troy |last2=Gilbertson |first2=Al |last3=Geurts |first3=Ron |last4=Green |first4=Barak |last5=Johnson |first5=Mary |last6=Reinitz |first6=Ilene |last7=Yantzer |first7=Phil | url=http://www.diamondcut.gia.edu/pdf/6_05_RDR_pg239_243pdf.pdf | archive-url=https://web.archive.org/web/20060829204314/http://www.diamondcut.gia.edu/pdf/6_05_RDR_pg239_243pdf.pdf | url-status=dead | archive-date=August 29, 2006 | title=Painting and Digging Out | journal=GIA Rapaport Diamond Report |date=June 3, 2005}}</ref> The single descriptive words are as follows: Excellent, Very Good, Good, Fair, and Poor.

Polish and symmetry are two important aspects of the cut. The polish grade describes the smoothness of the diamond's facets, and the symmetry grade refers to alignment of the facets. With poor polish, the surface of a facet can be dulled,dull and may create blurred or dulleddull sparkle. ItThe maystone constantlymay look like it needs to be cleaned. Because Withof poorthe lack of symmetry, light can be misdirected as it enters and exits the diamond.