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which
may be also increased to 64 or 72 or even 80 facets. All these facets
have been given definite names. The 58 facets are known as below:
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The
cutting of diamond in order to exhibit its optical properties to the
best advantage has been evolved upon long and practical experience. The
following relations are maintained in the cutting: Thickness TC from
the table to culet is 60% of the spread GG at the girdle. TO may vary
between 1/3 to slightly less than half of OC. Width of the table NM may
vary from 40-60% of the spread.
The
angles made by the main upper facets with the plane through the girdle
will be from 35-37°, whereas the lower main facets show an inclination
of about 41°. It has been found by experience that the maximum
brilliancy can be obtained when the incident and emergent rays are
perpendicular to the table. Several investigators have given the
angles and relationships that must be observed to obtain the ideal
brilliant cut for different gems. But to obtain the best effects,
brilliancy cannot be the sole criterion in cutting a stone, as the
ratio of the cut stone to the uncut stone has also to be considered. So
the data gathered from practical experience are found more valuable in
cutting stones.
In
a brilliant cut diamond, the light entering through the crown strikes
the lower facets at angles greater than the critical angle which is 24°
for diamond. So after repeatedly
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