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No Go Zone
We are now going to look in more detail at a zone of poorly performing crown and pavilion proportion combinations, or ‘no go zone’ (NGZ). There has been no previous mention in any literature of this most important issue. Tragically most of today’s diamonds are cut to proportions in this zone. The result is light leakage out the pavilion that can be seen through the table in a face up view. This zone includes many stones in the top cut grades of most labs cut standards.
Cutters will naturally cut to the maximum allowable crown and pavilion proportions used by institutes that issue reports on diamond cut standards in order to maximise yields and profitability. These diamonds display a dull area just inside the table and are defined here as no go zone diamonds (NGZ).
Figure 20. The first stone is a no-go-zone diamond (NGZ) which has AGS 0 proportions.
This combination of crown and pavilion angles allows some light entering the table to escape
out the pavilion (and vice-versa) resulting in an unattractive dull area just inside the table
when set. ‘Promotional’ proportioned diamonds like the second stone, unashamedly cut for weight
retention, can be more beautiful than many NGZ diamonds with top cut grades and higher prices.
The third stone scores the highest HCA score yet it is an AGS 3. (All three DiamCalc images
have 57% table sizes).
| Stone |
1 |
2 |
3 |
| Crown |
35.7° |
38° |
32° |
| Pavilion |
41.2° |
41.5° |
41° |
| HCA Score |
3.7 Good |
3.1 V Good |
0.5 Excellent |
This cause here is not related to the ‘Harding’ of ‘nail head’ resulting from a viewer’s head blocking the illumination source. Nailheads occur when pavilion angles are greater than 43.5° (Harding 1975). This darkness is caused by light leakage via the pavilion as a result the combined effect of increased crown and pavilion angles. If an observer were viewing a diamond in a face up position in the direction of the ray path then rather than observe light entering the diamond via the opposite side crown facet, the viewer ‘looks’ out the pavilion of the diamond.
In May 1999 I employed a competent and experienced appraiser and while testing her skills I asked her to rank the cut quality of a random parcel of ten diamonds. The results of this test were almost the opposite of my grading. I then understood the cause of the problem. Next day I left on an international buying trip and conducted similar tests with diamond dealers and merchants on three continents. Each time the preferred selection included diamonds that fell into the NGZ (I did not name it until 2000).
Experienced dealers grade unset diamonds with a loupe, backlit to enable a thorough clarity examination. Well-cut diamonds appears dark and dull because the head and loupe block light from the front of the stone, while NGZ diamonds sparkle because light from the dealer’s lamp is able to enter the pavilion. This and the fact that most institutions approve diamonds with NGZ proportions explains why we find we reject more than 90% of the diamonds we see.
Because less light is returned via the crown facets fire is enhanced when these stones are examined (in the manner that most dealers do) held at arms length in tweezers and rocked from side to side. The leakage area displays excellent fire, but once set of course there is no back lighting supply. This leakage not only results in a loss of light return, but also patchy static scintillation. NGZ diamonds should be set in ways that enable light to enter via the pavilion.
The HCA system penalises NGZ diamonds, and the more buyers that take heed, the sooner diamond cutters will respond to market demands and less NGZ diamonds would be cut.
The three DiamCalc images of the same identically proportioned stone show an AGS 0 or so called ‘ideal cut’. The diamond shown has very commonly found proportions of crown of 35.3° and a pavilion of 41.2°.
(This example is part of a letter to the Editor of the Gemmological Association of Great Britain concerning errors I believe Mr Michael Cowing made in his article published early in 2001. The Editor has refused to publish or enter into further discussions on the matter. The diamond used by at the top of Figure 11 and Table II of his article was an example of an ‘ideal cut’ diamond. However Michael appears to have inadvertently lit the pavilions of the diamonds from the side, there by obtaining completely wrong results. A true ideal cut diamond is little changed when illuminated from below the girdle.)
In realistic illumination where light is only supplied from above the girdle a large black donut shaped ‘hole’ can be seen inside the table. The ray path image shows the dull area is caused by light leaking out the pavilion.
By illuminating the right hand side of the diamond’s pavilion (the third illustration) with a strong virtual light bulb, light can be seen emerging from the area in question. The light has entered the pavilion in the reverse direction indicated by the ray path (for every action there is an equal and opposite reaction) and then refracts out through the black ‘hole’ making the right side of the diamond appear brilliant. A truly ideal cut diamond is little changed when illuminated from below the girdle.
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