To obtain maximum brilliancy from a gem, it must be fashioned in a manner that will cause the greatest possible amount of light entering the stone to be totally reflected from the pavilion facets and to pass out through the crown facets (Figure 19). This means that the light must strike the pavilion facets outside the critical angle cone and strike the crown facets within the cone. Since the size of the critical angle is related to the R.I. of the gem, every gem species requires a different set of angles to produce maximum brilliancy.
Few stones in the trade today are cut correctly, because few
lapidists have the necessary knowledge of optics to realize the
necessity for proper proportioning and most cutters, dealers and
retailers are more impressed by per carat price than they are by beauty.
By sacrificing correct proportions and retaining an excessive amount of
material in the pavilion of a stone, a greater total weight can be
recovered from a
given piece of rough, which means a lower price per carat. The
subsequent deepening and/or rounding of the pavilion, however, means
that much of the light striking the crown reaches the pavilion facets at
an angle within the stone's critical angle and thus passes, or "leaks"
out of the pavilion. In other words, when we look through the stone,
instead of observing brilliancy, we will see the mounting, the finger,
or the background over which the stone is being observed. In Figure 20,
for example, the lines on the paper are visible through the stone on the
right. Figure 21 illustrates two stones with the same critical angle.
Stone A is correctly cut for brilliancy, but stone B is cut with a
rounded pavilion to save weight. The advantages of correct cutting are
obvious.
