Natural Diamond Prices – December 2024
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Tabulation of the 15 stones presented as examples of 5 Categories
The article concludes with photos and proportion data of 15 stones given as examples of 5 Categories of cut quality. The Categories are presented as the ‘foundation’ of an eventual grading system, but a vague qualification is added; they may not be the basis of the final grades. It seems their foundation is incomplete which means that this analysis of it must also be incomplete.
In this review we will examine each of these stones using various tools designed and / or used every day by the author. There are advantages to reading on-line because there are several links included. However you may find having a printed copy at hand will improve comprehension, and be convenient for comparing images and discussions.
HCA proportions were chosen to cover the most widely available diamonds. The colored part of the chart shown below shows the pavilion on the vertical chart scale with a larger scale (39.5°-43°) because it is the proportion factor with the greatest impact on diamond appearance. The horizontal axis represents crown angle (28°-40°). The third most important variable, table size, is represented on 13 different charts ranging from 53% to 65%. In hind sight, it appears diamonds with shallower crowns and pavilions may become more common in the future.
In the chart shown here, an interpretation is made of some ranges of proportions given by the GIA based on my assumption that the GIA would concur with previous work done by MSU, Bruce Harding and Jasper Paulsen’s re-interpreted Tolkowsky calculations. But it should be noted that three of the 15 example stones fall outside these oval predictions.
The accuracy of the scanning devices used at the time of the GIA studies has resulted in significant rounding of table, crown and pavilion data. Pavilion lower girdle half facet length and crown star half facets length are quoted in 5% steps. All the proportions are considered as averages; angular deviations within the facet grouping for each measured parameter will be downgrade for diamonds with symmetry grades below Very Good. This approach is, in my opinion, less than perfect.
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| This HCA 55% table chart (extend to include some additional GIA proportions) has black ovals representing our interpretation of ranges of crown and pavilion angles for categories 1, 2 and 3 mentioned in the article. In white is the new AGS 0 ‘candidates’ proportions for 55% table. The black numbers represent the crown and pavilion (only) coordinates for each of the 15 stones in the 5 categories (except 4.3). |
Where additional specifications are shown in the charts below (next pages), it is because there was a difference between the relevant data quoted in the 2001 Fire article. The earlier information was not rounded as broadly as that in the Foundation article.
GIA Category 1
| RD01 HCA 0.6 | RD08 HCA 2.2 | RD20 HCA 1.9 | |||||||||
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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 0.61ct | GIA photo 0.50ct | GIA photo 0.62ct | |||||||||
| DiamCalc Scores | DiamCalc Scores | DiamCalc Scores | |||||||||
| Light Return Mono | 1.00 | Light Return Mono | 0.99 | Light Return Mono | 0.98 | ||||||
| Light Return Stereo | 0.99 | Light Return Stereo | 1.00 | Light Return Stereo | 0.98 | ||||||
| Contrast | 0.96 | Contrast | 0.97 | Contrast | 0.92 | ||||||
| Specifications | Specifications | Specifications | |||||||||
| Table Size | 54% | Table Size | 57% 58 | Table Size | 61% | ||||||
| Crown Angle | 34.0° (34.3°) [i] | Crown Angle | 33.5° (33.4) | Crown Angle | 34.5° (34.3) | ||||||
| Pavilion Angle | 40.6° | Pavilion Angle | 41.2° | Pavilion Angle | 40.8° (40.7) | ||||||
| Star Length | 50% (53.8%) | Star Length | 55% (54) | Star Length | 55% | ||||||
| Lower Girdle | 75% (81%) | Lower Girdle | 85% (84) | Lower Girdle | 80% (79) | ||||||
| Girdle Thickness | Thin-Med (2.9%) | Girdle Thickness | Med (3.9) | Girdle Thickness | Med (3.2) | ||||||
| Culet Size | None | Culet Size | None | Culet Size | Non | ||||||
| Total Depth | 61.2% | Total Depth | 61.1% | Total Depth | 59.6% | ||||||
| Polish | VG | Polish | VG | Polish | VG | ||||||
| Symmetry | VG | Symmetry | VG | Symmetry | VG | ||||||
| COMMENTS: GIA’s top grade appears looser than the proposed new AGS system | |||||||||||
| An obviously good choice; slightly shallower than Tolkowsky proportions and excellent symmetry (as shown in the photo) give the appearance of bigger facets, with bigger but less frequent flashes of fire. This is enhanced by short lower girdle facets (LG’s). | The weakest of the 3 stones. Shorter LG’s could improve the stone. LG’s and what appears as the weakest symmetry of the 3 stones adds to the appearance of more facets. Note: slight ideal-scope table leakage does not greatly reduce light return. |
The second best stone. Very few 61% table diamonds will also be given AGS’s top grade. The slight ideal-scope table leakage has little impact on light return. Note the thin looking pavilion mains caused by the larger sized table. |
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[i]This is additional data listed in (brackets). In the GIA G&G Fire article, Fall 2001, 28 stones are listed with precise numerals. The later Foundation article includes these stones in the 45 used in that study; however they have been rounded – crowns to 0.5°, pavilions to 0.2° and minor facets to 5% steps.
GIA Category 2
| RD16 HCA 0.7 | RD07 HCA 4.8 | RD03 HCA 2.4 | |||||||||||||
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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 0.82ct | GIA photo 0.76ct | GIA photo 0.55 | |||||||||||||
| DiamCalc Scores | Improved | DiamCalc Scores | DiamCalc Scores | ||||||||||||
| Light Return Mono | 0.98 | 1.00 | Light Return Mono | 0.82 | Light Return Mono | 0.97 | |||||||||
| Light Return Stereo | 0.94 | 0.98 | Light Return Stereo | 0.89 | Light Return Stereo | 0.97 | |||||||||
| Contrast | 0.91 | 1.03 | Contrast | 1.25 | Contrast | 0.90 | |||||||||
| Specifications | Specifications | Specifications | |||||||||||||
| Table Size | 53% 54 | 53% | Table Size | 53% | Table Size | 63% | |||||||||
| Crown Angle | 33.5° 33.8 | 33.5° | Crown Angle | 36.5° 36.4 | Crown Angle | 32.0° | |||||||||
| Pavilion Angle | 40.6° 40.4 | 40.6° | Pavilion Angle | 41.4° 41.5 | Pavilion Angle | 41.0° 40.9 | |||||||||
| Star Length | 50% 51.9 | 50% | Star Length | 55% 59.4 | Star Length | 60% | |||||||||
| Lower Girdle | 75% 76 | 85% | Lower Girdle | 90% 89 | Lower Girdle | 80% | |||||||||
| Girdle Thickness | Thin-Med 3.3 | 3.3 | Girdle Thickness | Thin-Med 3.1 | Girdle Thickness | Med-Stk 3.7 | |||||||||
| Culet Size | VSM | Culet Size | None | Culet Size | None | ||||||||||
| Total Depth | 61.2% | Total Depth | 64.1% | Total Depth | 58.6% | ||||||||||
| Polish | G | Polish | VG | Polish | G | ||||||||||
| Symmetry | VG | Symmetry | VG | Symmetry | G | ||||||||||
| COMMENTS: A pattern has developed; the second stone in grades 1 to 4 appear to be the worst | |||||||||||||||
| This stone was downgraded because of ‘Good’ polish. GIA claimed combined proportions make the pavilion mains look very dark; with a small table the lengthening the LG’s (to 85%) makes a diamond with these proportions perform very well. |
An interesting stone. Very high contrast and scintillation contribute to many small firey flashes; some are only visible because light return is poor. A contentious choice; if it were the same diameter as most of the smaller samples, would it have been selected? (See discussion) | The large table lets this stone down. Diamonds with large tables benefit from shorter LG’s, which add a little more contrast. ‘Good’ polish & symmetry also lower this stones grade. The table and upper girdle facets were said to be dark; this is not evident in the photo in G&G Fire, page 194 | |||||||||||||
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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GIA Category 4
This stone will show a fish eye and has very poor contrast.
Its light return is very good because it has very little leakage, but its pattern is unattractive.
The combination of shallow crown and pavilion angles increases the likelihood of chipping at the girdle.
This diamond will appear to be very much smaller than you would expect for its weight.
This is not just because of its smaller diameter – but more so because of the upper girdle leakage.
The table is dull because it acts like a mirror, reflecting back in the same direction – it is a nail head.
Described as a slight fish eye with “general darkness”, its main problem is a
lack of contrast and scintillation aided by a pronounced inclusion like fish eye.
Light return should be quite good.Had the LG’s been shorter by just 1% or 2%, fatter dark main facets would add contrast. Spread is just OK.
| RD19 HCA 7.5 (Fish eye) | RD33 HCA 10? (off scale) | RD37 HCA 6.5 (Fish eye) | |||||||||||
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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 0.72ct | GIA photo 0.64ct | GIA photo 0.50ct | |||||||||||
| DiamCalc Scores | DiamCalc Scores | DiamCalc Scores | |||||||||||
| Light Return Mono | 0.97 | Light Return Mono | 0.79 | Light Return Mono | 0.93 | ||||||||
| Light Return Stereo | 0.95 | Light Return Stereo | 0.82 | Light Return Stereo | 0.94 | ||||||||
| Contrast | 0.83 | Contrast | 1.00 | Contrast | 0.92 | ||||||||
| Specifications | Specifications | Specifications | |||||||||||
| Table Size | 62% 63 | Table Size | 56% | Table Size | 70% | ||||||||
| Crown Angle | 29.0° 29.2 | Crown Angle | 37.0° | Crown Angle | 33.5° | ||||||||
| Pavilion Angle | 39.6° 39.5 | Pavilion Angle | 44.0° | Pavilion Angle | 40.2° | ||||||||
| Star Length | 50% 51.7 | Star Length | 55% | Star Length | 60% | ||||||||
| Lower Girdle | 75% 76 | Lower Girdle | 70% | Lower Girdle | 80% | ||||||||
| Girdle Thickness | Med 3.3 | Girdle Thickness | Thn-Med | Girdle | Stk-Thk Bruted | ||||||||
| Culet Size | None | Culet Size | None | Culet Size | None | ||||||||
| Total Depth | 54.5% | Total Depth | 68.0% | Total Depth | 56.9% | ||||||||
| Polish | VG | Polish | VG | Polish | G | ||||||||
| Symmetry | VG | Symmetry | VG | Symmetry | G | ||||||||
| COMMENTS: The second stone is so bad it is hard to imagine the need for a lower grade? | |||||||||||||
GIA Category 5
| RD39 HCA 6.3 | RD43 HCA 6.2 | RD45 HCA 10? (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 0.70ct | GIA photo 0.50ct | GIA photo o.54ct | |||||||||
| DiamCalc Scores | DiamCalc Scores | DiamCalc Scores | |||||||||
| Light Return Mono | 0.88 | Light Return Mono | 0.77 | Light Return Mono | 0.65 | ||||||
| Light Return Stereo | 0.92 | Light Return Stereo | 0.81 | Light Return Stereo | 0.69 | ||||||
| Contrast | 1.03 | Contrast | 1.03 | Contrast | 0.99 | ||||||
| Specifications | Specifications | Specifications | |||||||||
| Table Size | 57% | Table Size | 57% | Table Size | 62% | ||||||
| Crown Angle | 35.5° | Crown Angle | 38.5° | Crown Angle | 37.0° | ||||||
| Pavilion Angle | 41.2° | Pavilion Angle | 41.8° | Pavilion Angle | 45.2° | ||||||
| Star Length | 55% | Star Length | 55% | Star Length | 60% | ||||||
| Lower Girdle | 80% | Lower Girdle | 80% | Lower Girdle | 85% | ||||||
| Girdle Thickness | Etk | Girdle Thickness | Thk-Vtk | Girdle Thickness | Med–Vtk | ||||||
| Culet Size | None | Culet Size | None | Culet Size | None | ||||||
| Total Depth | 74% | Total Depth | 71.5% | Total Depth | 69.3% | ||||||
| Polish | G | Polish | G | Polish | G | ||||||
| Symmetry | G | Symmetry | G | Symmetry | G | ||||||
| COMMENTS: This is a grade for the worst 1% of certified diamonds. (On 11/19/2004 there were only 441 plus 1.00ct rounds with depth % greater than 69% listed on Rapnet). | |||||||||||
| This one has old the AGS 0 proportions, but with an extremely thick girdle which adds 47% of additional weight. This 0.70ct diamond has the expected diameter of a 0.54ct diamond which has a ‘list’ of half the cost. |
Because of its very thick girdle, this 0.50ct diamond has the expected diameter of a 0.37ct stone, which has a ‘list’ price of less than half the cost. |
This is as bad as diamond cut quality gets. Stones like this are classed as ‘promotional’ and are seldom graded by laboratories. Its stated “large total depth” has nothing to do with its poor performance. (See discussion) | |||||||||
Categories 1 and 2
Category 1
The GIA have suggested five grades is the largest number of grades that they believe observers could discriminate. GIA’s top grade proportions appear looser than the top grade of the proposed new AGS system; these GIA category 1 stones would likely fall within the top 3 AGS grades of 0-2. Therefore after both GIA and AGS grading systems are implemented on grading reports, it is likely that the stricter AGS 0 system will continue to achieve a sellers premium as noted in the Pricescope Diamond Grading and Pricing Survey and reflected in business to business (B2B) web listings.
Category 2
RD07
This stones has very high contrast and scintillation; it appears to have many, very small firey flashes. Also because the stone has reduced light return, its firey flashes would not be more visible than in a stone with better light return. Indeed it could be hard to sell a diamond where a personal preference for fire would influence a buyer to accept a diamond with such poor light return.
If this stone were one of the smaller samples, would the size of some of its virtual facets have been too small for the survey observers to discern the flashes? This is one of the main problems with ‘parametric’ grading systems that apply the same grade to diamonds irrespective of their size.
This stone would also display an more apparent dead zone inside the table once worn and a film of grease has covered the pavilion. This would give the stone a similar effect to that of a Fish-eye because it shows the characteristic I have previously described as a No Go Zone. I would have given this stone a lower grade.
This DiamCalc ray trace shows the flash that appears in the upper table region on the GIA photo is within a leakage region. How were the diamonds illuminated for these photo’s? Was light able to enter the pavillions?This OctoNus ETAS map of reverse ray traced light return is for a model of RD07. It shows stronger than usual firey flashes on a virtual sphere. You can play with an early version of this software here. RD03 The table and upper girdle facets were said in the article to be dark; this is not evident in the photo of the same stone in G&G Fire, page 194. To test this we examined the DiamCalc light return scores which are broken down further into the table only; the ‘pop up’ window shows the table light return is very good in both static mode and when tilted through 30°.
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| This DiamCalc ray trace shows the flash that appears in the upper table region on the GIA photo is within a leakage region. How were the diamonds illuminated for these photo’s? Was light able to enter the pavillions? | This OctoNus ETAS map of reverse ray traced light return is for a model of RD07. It shows stronger than usual firey flashes on a virtual sphere. You can play with an early version of this software here. |
RD03
The table and upper girdle facets were said in the article to be dark; this is not evident in the photo of the same stone in G&G Fire, page 194. To test this we examined the DiamCalc light return scores which are broken down further into the table only; the ‘pop up’ window shows the table light return is very good in both static mode and when tilted through 30°.
Category 3
The first and last stones appear to be fair mid grade stones. The middle stone lacks observer’s presence contrast; I believe it would look worse than it appears in the photo.
RD22
This stone could possibly have been improved by the manufacturer; by lengthening the lower girdle facets and shortening the star facets (while holding the other proportions constant) would dramatically improve scintillation and would reduce the darkness caused by the short lower girdle facets which are the cause of the thick dark star in the table.
A second and better possibility may also have existed. Diamonds cut to these proportions are frequently the only stone polished produced from the rough. They are called ‘makeables’ because they are made without sawing from often rounded ‘dodecahedra’ rough. Of course we can not know if it this would have been possible – but often very steep crown angle diamonds can be made from such material. If it was possible in this instance, then making the crown angle 2° steeper would have resulted in a stone that scored HCA 1.2 FIC which is an abbreviation for Firey Ideal Cut. This stone would have a 4% better yield.
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| DiamCalc model – GIA data | Model with new minor facets | New minor facets & +2° crown | |
| Light Return Mono | 0.84 | 0.89 | 0.91 |
| Light Return Stereo | 0.83 | 0.92 | 0.93 |
| Contrast | 0.99 | 1.14 | 1.14 |
| Star facet length | 55% | 45% | 45% |
| Lower G’s | 75% | 90% | 90% |
In the article, GIA only mentions one stone as being a slight fisheye (RD37 Cat 4), but infact this stone, even though it has a small table size, would show a slight fish-eye (as would RD19 Cat 4).
RD11
If this survey had involved consumers actually purchasing diamonds, spread and apparent size would have been a very important factor in the grade setting of this and some other stones. This mid grade stone is a clear example of the difference between the institutional and the real worlds. The dark upper girdle facets in this stone (as mentioned in the article) are a result of leakage that is evident in its DiamCalc modelled ideal-scope image. Upper girdle facet leakage results in reduced ‘apparent size’ or ‘spread’ because our eyes loose the ability to distinguish the edge of the diamond; i.e. it looks smaller than its actual diameter. RD11 weighs 9% more than we would expect a nice diamond with this diameter to weigh, but a diamond with the same ‘apparent’ face up spread would about three quarters of the weight. This diamond would cost half as much again as a better cut diamond with a similar apparent size.
In my opinion ranking a diamond like this as a mid grade stone could result in retailers and consumers accusing GIA of pandering to large manufacturers and dealers, some donate generously to the GIA’s League of Honour Fund.
This diamond raises the possibility of a weakness in the lighting arrangement that GIA used. The DiamCalc ray trace shows that two facet sets are illuminated by light coming predominantly from the same light source. If we imagine placing this stone on a Brilliancescope®, with its annular or circular light source that moves up and down perpendicular to the diamonds table, we would expect the diamond to remain dull unless one of the 5 Brilliancescope® reading positions happens to fall in that region. In the DiamCalc ASET™ model, employing a variation on an Ideal-scope / Gilbertsonscope * approach, we see that the bright portion of the stone in the GIA photo are predominately pink.
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| The ray trace diagram shows that the center of the table, and most of the crown main facets, are both illuminated from the same direction; this shows as the red on the AGS ASET DiamCalc lighting model. The stone would appear dead if light from those directions was unavailable. (The ASET blue illumination would be largely obscured by an observers head. The green colour indicates light that has come from close to the horizon, the least likely source of bright illumination.) |
RD06
GIA’s computer model predicted lower crown angle stones would have better brilliance; naturally they needed to test this with visual observations; stones were polished especially for this purpose. The computer scientists must have been disappointed by the results. On page 225 two reasons are given for mediocre performance. Firstly they mention low values for crown height and angle and secondly they refer to the stones lack of contrast and scintillation. I believe they made the pavilion angle too shallow. (It would be good if GIA stopped referring to “Crown Height”. A diamonds crown height is determined by two factors; crown angle and table size. Crown height per se has little or no determining impact on a diamonds appearance.)
A low crown height needs to be combined with an appropriate pavilion angle. GIA predictions of high light return for RD06 (WLR 3.01) in their 1998 G&G light return study failed to account for an observers head, a fact well known and discussed adnauseum. Also well known is the head obscuration of shallow crown stones has a more deleterious impact than with other proportion sets. Because they made the pavilion too shallow f particular crown height, the stone shows what all shallow stones show – too much darkness. The darkness could have been reduced with adjustments to minor facets, but a better approach would have been to cut a deeper pavilion as in the example below (A pavilion angle in between would probably have yielded a more balanced stone). It appears that GIA have failed to heed the feedback from critics and have not made appropriate adjustments to their brilliance software.
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| The stone on the left is a DaimCalc model of the GIA stone. On the right is this stone with similar proportions, but 1° deeper pavilion; it scores an amazing 1.11 and 1.08 for DiamCalc light return, but rates only 0.75 for contrast (questioning the Brilliancescope® results for scintillation). |
GIA have made no mention that shallow crown angle is a durability issue. Perhaps GIA will follow AGS’s lead and downgrade shallow crown angles for durability in the finalized system?
* Al Gilbertson is now one of the GIA Cut Study team
Categories 4 and 5
GIA-GTL would be under considerable pressure from benefactors and clients who submit large quantities of diamonds for grading to be liberal one the one hand. On the other hand they are an ethical and honourable institution charged with protecting the public and promoting the industry. The second stone (RD33) in this category is so poor that it is hard to imagine the need for a lower grade?
RD37
Worthy of note: The Liddicoat depth estimation is a commonly used and GIA taught method of pavilion depth estimation. By estimating the width of the table reflection compared to the width of the table, a reckoning of the pavilion depth % is made (add 1/10th of the % to 40 = estimated pavilion depth). In this case the table reflection is 1/3rd the table width. This would lead us to believe the pavilion depth is 43.3%, but the depth is actually only 42.3%. The Rapid Sight method works with a relatively small range of table sizes and is also affected by girdle thickness and crown height. In this case a 1% over estimation of depth percentage would indicate the stone had a 40,9° pavilion; knowing the pavilion was only 40.2° might influence a buyers decisions.
Category 5
I question the validity of this grade that would apply to the worst 1% of certified diamonds based on a survey conducted on Rapnet on 11/19/2004. There were only 441 plus 1.00ct rounds with depth % greater than 69% listed out of approximately 40,000 round stones searched by the same criteria. Surely the lowest grade in a 5 grade system should be set to reflect around 10 to 20% of the goods in the market?
It would also have been instructive to see more than one example of a diamond downgraded for other than having an overly thick girdle.
The optical performance of RD39 appears to be as good, or better, than some stones in Categories 3 and 4. Aside from the extremely thick girdle (which does not have a major impact on optical performance) the crown and pavilion angles represent the steepest and deepest AGS 0 angles; was the inclusion of this stone a political choice?
Discussion
The GIA study noted during the observational survey that factors other than proportions played a role in diamond beauty. This table attempts to simplify those observeations.
| Factor | No effect | Slight effect | Noticeable reduction |
| Color | D decrease in apparent brightness as saturation increases Z | ||
| Clarity | IF to SI1 | Grade determining Clouds in SI2 | Grade determining Clouds in I1 > |
| Symmetry | Excellent VG Good | Fair > reduced brightness > Poor | |
| Polish | Excellent VG Good | Fair> reduced brightness and fire > Poor | |
| Girdle | No effect on apparent brightness and fire from girdle condition | ||
| Fluoro | No effect on apparent brightness and fire | ||
Factors that showed discernable effects on brightness and fire that were noted by observers during the GIA survey study.
Conclusion
There are a number of instances in the paper where references are made to diamond performance or appearance based on percentages or preconceived trade based ideas of how a diamond should look, or what proportions are considered ‘acceptable’. On page 220 they state “For example, we downgraded diamonds with pavilion angles that were very shallow or very deep because these proportions generally changed the face-up appearance of the diamond in ways that made it less desirable to experienced trade observers.”
The findings of this GIA study, and other studies and approaches to cut grading, will fundamentally change industry expectations. If part of the purpose of this GIA article is to change those preconceptions to match the findings of human observers and the realities of the laws of physics, why be considerate of preconceived trade customs and opinions? As Paul Slegers says, we should not be led by logic dictated by adages like “my child is beautiful”. This would lead to a lost opportunity to take full advantage of the laws of nature to maximise both the beauty and the yield on each and every one of those “finely crystallized carbon” creations.
Paul also notes that trade observers are educated in a certain way of looking, judging and thinking. No matter how ‘blind’ you make their observations by not giving them any parameters before judging, they can identify certain parameters and they may inevitably be influenced in their observational judgements.
This GIA observation based study used diamonds with considerably different weights and diameters; I believe it is un-reasonable to compare diamonds with noticeably different diameters. Diameters were not published, but by estimation with DiamCalc using published proportion data, they ranged from approximately 4.7mm to more than 6mm. Some of the 15 example stones included heavier diamonds that had larger spreads, and some of the deepest diamonds had the lightest weights.
There appears to be no part of the study that included apparent or observed differences in spread. Stones like RD11 in Cat. 3 and RD33 in Cat. 4 would appear significantly smaller than their measured diameters because of greater upper girdle leakage. In my experience this is a major factor in diamond desirability; it would result in diamonds of these grades being rejected in favour of stones with a lower GIA grade but larger apparent size.
For all practical purposes, GIA has designed a 4 Category grading system. The fifth Category may only apply to the worst 1% of certified diamonds. It could be argued that there is a larger percentage of overly deep uncertified ‘promotional’ grade diamonds, but one can not help wondering if this category was set to appease some sectors of production and wholesale within the industry? Does it serve retailers and consumers to propose an irrelevant fifth grade when there are stones in the fourth category that could already be considered by many as only a little better than unpolished rough diamonds? Cut quality is confusing and creates doubts in consumer’s minds. Doubts are impediments to buying; that inevitably reduce the growth of diamond jewelry as a whole. Brighter, more firey and sparkly diamonds sell themselves and keep customers coming back for more.
It would appear the basis of the GIA grading system will be the proportion data from a scan. Such a system can be described as a parametric grading system that employs look up charts similar to HCA (but with additional minor facet proportions and symmetry and polish grades). The AGS will shortly introduce a more advanced system using parametric and direct assessment techniques. Neither system appears to account for differences in diamond size and accompanying effects on appearance. Neither system is readily functional for the designing and planning of polished diamonds by the scanning software that is widely employed in the industry for rough diamonds. The manufacturing industry will discover that adapting to these new grading systems is rather difficult. OctoNus is continuing with the development of a 3D software based grading system. This system will account for diamond appearance based on size differences, the same system will work for any shaped diamond and it will function in reverse by enabling the best yield planning and the production of rough diamonds into the most attractive gem possible. Eventually this approach to cut grading could do away with the encumbrance of predetermined faceting arrangements; imagine non symmetric one of a kind diamonds with optimal beauty?
Symmetry and averaged data
It appears from the articles and public presentations in Basel and Hong Kong that the GIA system will use averaged crown and pavilion angle data. Lower symmetry grades will lead to lessor overall cut grades. There are at least three shortcomings from this approach.
Firstly consider the idea that there can be ‘sweet spots’ within a grade. If all the measured proportions are such that the stone is well within grade boundaries, then symmetry deviations may have little impact on the diamonds appearance. The GIA authors have qualified this by the fair claim that a diamond in the top grade should also meet certain crafting quality standards. But consider a diamond that is near the boundary of a proportion grade that has certain types of symmetry deviations, like for example, a slight squarish out of roundness that results in the combined effect of steeper crown facets directly over deeper pavilion angles. The additive effect of this type of small symmetry deviation on the ray paths in a diamond can result in far greater leakage and reduced light return. AGS would be able to downgrade such stones because of their combined use of parametric and direct determination using their 3 colored ASET scope.
Secondly, the GIA article mentioned that diamonds that exhibited excellent symmetry with Ideal-Scope or Hearts and Arrows viewers were not rated any higher by observers than those with lessor patterns. I suspect that their study only focused on diamonds within very good proportions, and in this respect, Peter Yantzer and I have never subscribed to the idea that Hearts and Arrows diamonds offer any better appearance to the naked eye than well cut diamonds with small symmetry deviations. Had GIA tested diamonds with the ideal-scope with the purpose that it is designed to be used for, rather than simply as a gauge of symmetry, I feel they would have discovered benefits form its usage and weaknesses in their current symmetry grading method.
 |
| These stones have a 1% out of round squarish profile, yet they would be graded as having excellent symmetry by all labs. The stone on the left – four of the eight sets of main facets have crown and pavilion angles of less than 34.9° and 40.9°, while the section that is leaking with a less well defined star, has crown and pavilion angles of 35.1° and 41.2°. The HCA scores for each set of proportions are 1.7 and 3.4 respectively. The stone on the right is the same model with a one half degree shallower pavilion; it is in a “sweet spot” with HCA scores of 0.7 and 1.4 respectively. |
Thirdly, another disadvantage of a parametric system is that it takes no account of a diamonds size. GIA will give the same grade for different sized round brilliants with equal proportions[ii] . Tom Moses acknowledged that a 1/3rd carat diamond with equivalent parameters would have a different appearance to a 5ct diamond. Better symmetry in a well cut 1/3rd carat diamond aids in producing easily discernable firey and contrasting facets. But in larger diamonds, of say 5 carat or more, some types of symmetry deviations can actually improve a diamonds appearance[iii] by increasing the frequency of flashes from an observably larger number of virtual facets, even though both stones have the same number of facets.
The current methods for grading symmetry has evolved from the basis of the capacity to assess or detect deviations. These systems can be shown to be inadequate for the current task.
Rounding errors
As noted, the GIA has applied a rounding of parameters, presumably because of the inherent accuracy of existing scanners. Rounding introduces errors. Accurate measurements of azimuth are also critical for parametric grading when considering the Indexing; Sarin and Ogi scanners appear to have limitations in this area.
Upper and Lower Girdle Facet Indexing
GIA published an article 13th Feb 2004 discussing azimuth and angular adjustments or minor facets that are achieved with the aid of ‘indexing’. The accounts of the affect of these polishing practices on diamond appearance ran counter to the widely held opinions of others in this field. For instance diamonds that have painted or built up upper girdle facets like EightStar diamonds® were said to show “little contrast except for the dark appearance of the pavilion mains beneath the table”. The current article makes no mention of how painted or dug out facets will be interpreted in a new grading system, nor of how these azimuth shifts could be detected with adequate accuracy.
[ii] I asked about this on behalf of Sergey Sivovolenko from OctoNus at the GIA Basel Gemfest 2004 presentation. Tom Moses replied that they would give the same grade to a 0.30ct diamond and a 5ct diamond if all proportions etc were the same.
[iii] Personal communication with Pol Van der Steen (DiamCad, Belgium) who is a specialist manufacturer of very large diamonds.
Future Pricing
The issue of the effect on pricing of diamonds was not mentioned in the GIA Foundation article, however I feel that this is one of the most interesting aspects from most readers perspective. This is my opinion; the market currently has a comparatively narrow band of price differentials for variations in cut quality. For instance dealer to dealer 1.00 ct round brilliant Rapaport pricing can range from ‘list’ down to 50% discount; put simply a good cut can cost twice as much as a bad cut. Compare this to Clarity and Colour Rapaport pricing; the average difference between IF and I3 is 9 times and for D to M the figure is 4 times.
What will the market do with this new information? It is likely that over time a new market pricing will evolve. During the year after GIA’s system is released I predict that the gap will widen to 2.5 times. Over a longer period 3 to 4 times (67-75% discount) could be common as poor graded goods become more difficult to sell. Consumers will become more aware of the need for a grading report with a reliable and respected cut grade.
| Possible GIA terminology | Discount to the next lower grade | Short term prediction | |||
| 10% | 15% | 20% | 25% | ||
| Excellent | 100 | 100 | 100 | 100 | 100 |
| Very Good | 90 | 85 | 80 | 75 | 75 |
| Good | 81 | 72 | 64 | 56 | 60 |
| Fair | 72 | 61 | 51 | 42 | 50 |
| Poor | 63 | 52 | 41 | 32 | 40 |
Finally, it is our opinion that the foundation of the GIA system is not built on rock hard science. This most recent article is verbose and vague in many areas. But it appears that by and large the GIA is moving forward.
Acknowledgements
The following people have contributed ideas and or reviewed and made suggestions to this review:
Paul Slegers
Yuri Shelementiev
Sergey Sivovolenko
Pol Van der Steen
Peter Yantzer
Leonid Tcharnyi has spent hours formatting and putting it here on Pricescope.
Thank you all.
Related Links
Article Series
This article is part 1 of a 3 part series. Other articles in this series are shown below:
- A review of the “foundation” of GIA’s new cut grade system
- GIA Excellent Cut Grade: Case Study
- Letter to the Editor of the Australian Gemmologist
