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| Diamond Tutorial | Holloway Cut Adviser |
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The 'X' on the chart below marks the pavilion° / crown° position on the closest table size chart. The white outline shows the AGS ideal ‘candidate’ range and the green dotted line shows the GIA Excellent ‘candidate’ range. A conservative (but more costly) choice would be a diamond with proportions in the overlap of both AGS and GIA ideal and excellent ranges. Holloway Cut Adviser was launched on Pricescope in 2001 after 15 years study and lecturing diamond cut. I developed HCA with the Firescope and DiamCalc. HCA is only for round diamonds with known Table, Crown, Pavilion and Total Depth inputs; it has no idea about the diamonds symmetry, poor minor facet proportions or various cutters tricks. Use it to reject known poor performers and narrow down your selection. Please do not use it for final selection; it will not replace an analysis with an Ideal-scope and/or an independent appraisal. Stones listed here are ranked by HCA scores, and many have ideal-scope photo's and other cool stuff. GIA's new system is similar to HCA but with symmetry, polish and minor facet data, but the data on GIA reports has been severely rounded. In order of importance: Pavilion 0.2°, Crown 0,5° lower girdle depth 5%. Steeper crown and deeper pavilion GIA graded 'Excellent' diamonds have a dark ring in the table area and will show the dirt on the pavilion. Diamonds that rate below 2 (red on the chart) are unlikely to show too much leakage darkness, overly thin or thick girdles, or fish-eyes. But there are other negatives that HCA can't predict. Additionally, if you examine diamonds from closer up than 10 inches (25cm) in your selection process, you will probably prefer diamonds with proportions that fall in the “Young Peoples Rings” area that is marked on the chart below in the upper right of the red zone. Figure. This chart indicates some diamond proportions and information that you may find useful.
 Shallow stones (lower left on the chart) look darker if you have excellent close up vision because your head obstructs more light sources which makes a shallow diamond appear darker. But shallow diamonds have a bigger spread, and are great for pendants and earrings, where normal social viewing distances apply. Stones near the center of the red region (the lowest scores) are least affected by symmetry variations. Alternatively hearts and arrows diamonds, which have excellent optical symmetry, but often HCA scores around 2, may out-perform diamonds with lesser symmetry and lower HCA scores. Deeper stones in the upper green and blue zones have more leakage and often appear dark just inside the table and the outer girdle edges; they are best set in open backed rings so light can get in the bottom or pavilion. But small 'vee' shaped leakage zones near the outer edges of a diamond, as seen with an Ideal-Scope, can add to a diamonds contrast and brilliance. Trivia HCA caused a bit of a stir when we first offered it free on Pricescope in 2001; like any new concept that affects diamond sales, HCA was not liked by many vendors. GIA spent millions on cut research, Octonus and Moscow State University used their brains, and AGS did a seat of the pants study like HCA. All support the inverse Crown and Pavilion theory. HCA is fine-tuned from time to time - results may change without notice. Remember - only use HCA to reject diamonds & narrow down your selection. A full technical description of HCA can be found at www.diamond-cut.com.au Garry Holloway, FGAA DDT Discuss on the Forum | |
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