| A review of the “foundation” of GIA’s new cut grade system |
| by Garry Holloway |
Published
30/11/2004
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Diamond Grading
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GIA Category 3
| RD22 HCA 2.4 | RD11 HCA 4.1 | RD06 HCA 4 ? (off scale) | 
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| Ideal-Scope Lighting | Ideal-Scope Lighting | Ideal-Scope Lighting | | | | 
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| Jewellery Shop Lighting | Jewellery Shop Lighting | Jewellery Shop Lighting | | | | 
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| Disco Lighting | Disco Lighting | Disco Lighting | | | | 
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| GIA photo 0.81ct | GIA photo 0.71ct | GIA photo 0.59ct | | | | DiamCalc Scores | DiamCalc Scores | DiamCalc Scores | Improved | Light Return Mono | | 0.83 | | Light Return Mono | | 0.81 | | Light Return Mono | 0.99 | 1.04 | Light Return Stereo | | 0.82 | | Light Return Stereo | | 0.84 | | Light Return Stereo | 1.00 | 1.04 | Contrast | | 0.98 | | Contrast | | 1.09 | | Contrast | 0.88 | 0.89 | Specifications | Specifications | Specifications | Table Size | | 54% 55 | Table Size | | 58% | Table Size | 56% 57 | 56% | Crown Angle | | 35.5° 35.9 | Crown Angle | | 37.0° 37.2 | Crown Angle | 23.0° 23.1 | 23.0° | Pavilion Angle | | 39.4° 39.2 | Pavilion Angle | | 42.2° 41.9 | Pavilion Angle | 42.0° 41.9 | 42.5° | Star Length | | 55% 54.2 | Star Length | | 45% 49.1 | Star Length | 60% 60.6 | 45% | Lower Girdle | | 75% 77 | Lower Girdle | | 85% 87 | Lower Girdle | 80% 78 | 85% | Girdle Thickness | | Thin 3.3 | Girdle Thickness | | Med-Stk 4.3 | Girdle Thickness | Med-Stk3.2 | | Culet Size | | None | Culet Size | | None | Culet Size | None | | Total Depth | | 60.6% | Total Depth | | 64.9% | Total Depth | 57.2% | | Polish | | VG | Polish | | G | Polish | VG | | Symmetry | | VG | Symmetry | | VG | Symmetry | VG | | COMMENTS: The first and last stones appear to be fair mid grade stones. The middle stone lacks observer’s presence contrast; it would look worse than it appears in the photo. | “..somewhat shallow pavilion angle produces dark pavilion mains”. As with Cat. 2 stone RD16; longer LG’s (90% & 45% stars) improves table brightness and scintillation. A 2° steeper crown would have even better light return, fire and a 3% better yield and HCA of 1.3. The stone is nearly a fisheye. | Should this be a mid grade stone? Upper girdle facet leakage will result in an apparent size of a diamond 2/3rds its weight. Bad leakage helps fire and scintillation (appearance of having more facets). The leakage and lack of contrast from observer obstruction reduces brilliance. | GIA’s computer model predicted good brilliance for lower crown angle stones; they had to test it. With a 0.5° deeper pavilion, 45% stars and 85% LG’s we get a stone worthy of Cat. 2 and a better yield. To prevent chipping, very shallow crown diamonds should have a slightly thicker girdle. (See discussion) |
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