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Faceting limits

By Bruce L. Harding
Holden, Massachusetts, USA
Copyright GIA
Charts for Specific Materials table of contents
  Reference 1 and 2 3 Zone A Zone B Zone C
quartz 43/42 43/40-50 43/37 --- ---
beryl 43/42 43/40-50 43/35 40/34 40/18
topaz &
39/43 (40/40) 43/33 40/34 40/17
42/41 39/37 39/23
peridot &
(40/40) 43/32 40/32 39/22
42/39 39/36 38/27
spinel &
42/37 (40/40) 43/31 40/31 39/21
42/37 39/35 38/26
corundum & garnets 42/37 (40/40) 43/30 40/30 39/20
42/36 39/33 38/25
  38/36 37/30
low zircon & YAG 41/40
(40/40) 43/28 40/28 39/20
42/34 39/31 38/24
  38/34 37/28
diamond 40.75/34.5
  43/20 41/18 38/20
42/26 40/21 37/20
  39/24 36/20
* 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.
Comments table of contents
     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.
© Octonus Software & MSU Gemological Center.