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Color
This
effect can be achieved because light entering a gem
is refracted, or bent. But the angles at which the facets
are cut are very important. If the angles are wrongly
matched to the gem's refractive power, light will not
be returned to the viewer's eye. Instead it may pass
directly through, or be reflected off to the side.
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The
result is a gem that appears "dead" and lifeless.
The term "fisheye" is sometimes used where
the central portion of the stone lacks brilliancy. Factors
that affect gem brilliancy include facet angles, transparency,
the perfection of polish on facet surfaces, and the
proportions of the gem. |
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The
propor¬tions of a gem are the relative sizes of the crown,
or upper portion, pavilion, or lower portion, and diameter
measured at the girdle. The diameter of the table, or large
top facet, is also important, as well as the overall height
of the stone compared to the diameter.
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DIAMONDS,
GEMSTONES & CRYSTALS
FACTS & FIGURES |
| Brilliancy
and dispersion are most important in the case
of diamonds. For centuries diamonds were cut primarily
to maximize weight and size, and the scientific
principles of cutting were not yet understood.
Today
many shops cut for maximum brilliancy and overall
appearance. Many commercial diamonds suffer in
attractiveness be¬cause of oversized tables,
thin crowns, or over deep pavilions.
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An
expert can often tell which of these problems is present by
the appearance of the stone to his eye. Since both maximum
brilliancy and maximum dispersion cannot be achieved with
the same set of angles and propor¬tions, an "optimum"
set of parameters has been worked out. Stones cut to these
proportions and angles achieve high brilliancy and a great
amount of dispersive "fire." |
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| diamond
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