d
Ruskin's viewpoint, nevertheless the art of man can make use of the
optical properties of transparent minerals, properties no less wonderful
than those exhibited in crystallization, and indeed intimately
associated with the latter, and, by shaping the rough material in
accordance with these optical properties, greatly enhance the beauty of
the gem.

No material illustrates the wonderful improvement that may be brought
about by cutting and polishing better than diamond. In the rough the
diamond is less attractive in appearance than rock crystal. G. F.
Herbert-Smith likens its appearance to that of soda crystals. Another
author likens it to gum arabic. The surface of the rough diamond is
usually ridged by the overlapping of minute layers or strata of the
material so that one cannot look into the clear interior any more than
one can look into a bank, through the prism-glass windows that are so
much used to diffuse the light that enters by means of them. Being thus
of a rough exterior the uncut diamond shows none of the snap and fire
which are developed by proper cutting.

As the diamond perhaps shows more improvement on being cut than any
other stone, and as the art of cutting the diamond is distinct from that
of cutting other precious stones, both in the method of cutting and in
the fact that the workers who cut diamonds cut no other precious stones,
it will be well to consider diamond cutting separately.

Before discussing the methods by which the shaping and polishing are
accomplished let us consider briefly the object that is in view in thus
altering the shape and smoothing the surface of the rough material.

HOW CUTTING INCREASES BRILLIANCY. Primarily the object of cutting a
diamond is to make it more brilliant. So true is this that the usual
form to which diamonds are cut has come to be called the _brilliant_.
The adjective has become a noun. The increased brilliancy is due mainly
to two effects: First, greatly increased reflection of light, and
second, dispersion of light. The reflection is partly external but
principally internal.

Taking up first the internal reflection which is responsible for most of
the white brilliancy of the cut stone we must note that it is a fact
that light that is passing through any transparent material will, upon
arriving at any polished surface, either penetrate and emerge or else it
will be reflected within the material, depending upon the angle at
which the light strikes the s