Faceting limits
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By Bruce
L. Harding
Holden, Massachusetts, USA
Copyright GIA |
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Figure 9
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Internal rays which hit the table
at more than the critical angle to its normal will be reflected
back into the gem and probably be lost through the pavilion.
This can be avoided by limiting the table size so that any such
rays hit the bezel instead of the table, Figure 9 shows the
two extreme rays which cause this problem; oddly, they both
define the same maximum table size.
Calculations show that this maximum
table size is seldom less than 75% of girdle width for most
stones faceted by amateurs (it is 53% for diamond with standard
slopes). |
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Figure 10
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The only justification for a minimum
table size is to prevent seeing reflections of the culet in
the bezel when looking perpendicular to the table. Figure 10
shows that this limit is defined by a ray from the culet to
the edge of the table which is bent perpendicular to the table
by the bezel.
Calculations show that this minimum
table size is seldom more than 50% of girdle width. |
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Charts of maximum and minimum
table widths, according to these criteria, will be presented
vs. pavilion and bezel slope combinations for various refractive
indexes in a future article. Another sequel will show that most
of the light returned to the viewer passes through the bezel,
so this in an argument against large tables.
Temporarily, therefore, table
widths from 50% to 60% should be considered ideal until more
data or criteria are available. |
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