Jess Dispersion is highest when the exiting light ray (coming
from the pavilion to exit through the crown) is as near to the
angle 23 degrees and 56 minutes as possible without exceeding
that angle (which then reflects back into the stone when
exceeded). It is the combination of angles of the crown and
pavilion and perfect alignment which maximize this.
However, when dispersion is extremely high a new problem occurs.
Since dispersion is white light being broken into spectral
colors, we have colored light being viewed overhead which is not
as bright as white light. Diamond is a very efficient reflector
of white light–but when there is high dispersion, the effect
makes the stone appear darker than a diamond with very good
light return but little dispersion. “Eight star” diamonds are a
good example of this–they all had very high measurements of
dispersion, but appear darker to many when viewed face up next
to other “ideal” cuts. This is because they deliberately “push
the diamonds to the edge” to maximize dispersion. Diastar
(Hearts on Fire) does not do this–they instead focus on
minimizing light leakage and maximizing symmetry. This allows
maximum light return–which is usually very bright (unless the
stone has a high level of dispersion–which is not planned).
The discussion about dispersion and light return related to
ideals is based on an 8-main symmetry. Other facet arrangements
will produce new combinations of scintillation which can add
beauty to the diamond’s appearance. The term “ideal” is a
traditional 8 main symmetry with 57/58 facets whose angles
approxiamte those suggested by Tolkowsky. Scintialltion may not
be nearly as great as some of the newer facet combinations.
Etienne Perret made a valid point. Different facet arrangements
will have different results --the critical issue being that
light leakage is minimized and light is efficiently returned to
the viewer (one could arrange for light to not leak, yet not
return it dirrectly to the viewer and the stone would be dead).
Careful symmetry maximizes lights ability in this issue. Al
Gilbertson