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Faceting limits
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By Bruce
L. Harding
Holden, Massachusetts, USA
Copyright GIA |
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pavil/bezel slopes P/B |
COMMON RECOMMENDATIONS |
RECOMMENDED PER CHARTS |
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Reference |
1 and 2 |
3 |
Zone A |
Zone B |
Zone C |
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quartz |
43/42 |
43/40-50 |
43/37 |
--- |
--- |
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beryl |
43/42 |
43/40-50 |
43/35 |
40/34 |
40/18 |
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topaz &
tourmaline |
39/43 |
(40/40) |
43/33 |
40/34 |
40/17 |
| 42/41 |
39/37 |
39/23 |
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peridot &
spodumene |
(40.42)
39/43 |
(40/40) |
43/32 |
40/32 |
39/22 |
| 42/39 |
39/36 |
38/27 |
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spinel &
grossular |
42/37 |
(40/40) |
43/31 |
40/31 |
39/21 |
| 42/37 |
39/35 |
38/26 |
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corundum & garnets |
42/37 |
(40/40) |
43/30 |
40/30 |
39/20 |
| 42/36 |
39/33 |
38/25 |
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38/36 |
37/30 |
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low zircon & YAG |
41/40
42/37 |
(40/40) |
43/28 |
40/28 |
39/20 |
| 42/34 |
39/31 |
38/24 |
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38/34 |
37/28 |
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diamond |
40.75/34.5
(Tolkowsky)4 |
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43/20 |
41/18 |
38/20 |
| 42/26 |
40/21 |
37/20 |
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39/24 |
36/20 |
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| * Zone C includes the Table as a bezel with
zero slope. Experimental cuts in this zone are brilliant
but look strange and may not be desirable |
Faceting charts are shown for
eight different refractive indexes representing common faceting
materials: for other materials, use the chart with the closest
refractive index, or make your own if the RI is over 1.9. Marks
on these charts represent designs recommended by three common
references: a solid dot indicates references referred to as
1 and 2; a circle indicates another reference 3.
The table on this page summarizes
these data and indicates designs which appear to be best according
to the charts. Designs which the chart indicates as not recommended
are shown in parentheses. |
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The charts agree with references
1 and 2 in all examples but one and disagree with reference
3 in all but part of two. Reference 3 recommends the same angles
for many stones; for RI`s greater than 1.6 these designs give
almost zero divergences for table-to-bezel rays – perhaps this
was the author`s intent, being unaware of the viewer`s head
problem.
If it were important to find one
design which was suitable to all stones, the closest solutions
according to these charts would be: P/B=43°/35° (Zone A), =40°/32°
(Zone B), =40°/17° (Zone C).
It is interesting to note that
there are no good designs for the pavilion slope of 41°.
It is also interesting to note
that pavilion slope is more critical than bezel slope; that
is why I always cut the pavilion first (in addition to other
reasons) if I run short of material I can always «fudge» the
crown a bit.
Zones A, B and C are of particular
interest in re-cutting. A worn stone cut per Zone A can be recut
per Zone B, and again per Zone C, always retaining the same
girdle so that it can be replaced in the same setting; beauty
will be retained and weight loss will be minimized. I have done
this and its works!
Although it is possible to design
a gem where the viewer`s head is not an obstruction, the viewer`s
body is another problem and will always produce a dark area
in the gem. A sequel will show that this dark area is most noticeable
when the gem is cut in Zone C. |
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| © Octonus Software & MSU Gemological Center.
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