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Faceting limits

By Bruce L. Harding
Holden, Massachusetts, USA
Copyright GIA
 
Maximum Table Width table of contents

Figure 9

     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).
 
Minimum Table Width table of contents
 

Figure 10

    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.
 
Optimum Table Width table of contents
     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.
 
Octonus Software & MSU Gemological Center.