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Faceting limits

By Bruce L. Harding
Holden, Massachusetts, USA
Copyright GIA
Range of Reflections Through Table table of contents
Figure 7a
(click image for big view)
Figure 7b
(click image for big view)
          Figure 7A shows the extreme angles of a ray reflecting off both sides of the pavilion: min = C-P (negative) and max = 180°-C-3P (because D = 180°-4P). The corresponding external angles fmin and fmax are the limits at which the viewer can see reflections through the table from the far and near sides of the pavilion, respectively. Angle fmin is therefore the limit for seeing reflections from both sides at once.
     These limits are noted across the top of the faceting chart in Figure 7B for each pavilion slope. Note that as one limit increases, the other decreases.
«Live» Center table of contents
Figure 8a
          For ultimate liveliness it should be possible to see reflections in the table from both sides of the pavilion with both eyes at once. At a viewing distance of one foot, as shown in Figure 8A, the angle between reflections to both eyes (two different rays) is about 12°. To see reflections from both sides with both eyes, therefore, the minimum external table reflection angle fmin must be at least 6°.
     Designs which do not satisfy this condition are shaded light at the left of Figure 8B.
Faceter`s Options table of contents
Figure 8b
(click image for big view)

     The faceter should choose a design from one of the lighter areas of the chart according to that he thinks is «best»

  1. increase pavilion slope for wider range of «live» center,
  2. decrease pavilion slope for wider range of table reflections,
  3. increase bezel slope for greater dispersion,
  4. decrease bezel slope for greater brightness.
© Octonus Software & MSU Gemological Center.