n worn they
soon lose their polish and with the loss of polish they lose their
beauty. The attack of the gases before mentioned darkens the surfaces of
the imitation and further dulls it. When fresh and new a well cut piece
of colorless paste has a snap and fire that approaches that of diamond.
The surface luster is not adamantine, however, and the edges of the
facets cannot be polished so sharply as those on a diamond. Moreover the
refractive index, while high, is never so high as in a diamond and hence
the brilliant cannot be so shaped as to secure the amount of total
reflection given by a well-made diamond. Hence, the paste brilliant,
while quite satisfying as seen from squarely in front, is weak and dark
in the center as seen when tilted to one side. By these differences the
trained eye can detect paste imitations of diamond at a glance without
recourse to tests of specific gravity, hardness, etc.

Pastes, being amorphous, are singly refracting, as is diamond. This fact
helps the appearance of the paste brilliant, for light does not divide
within it to become weakened in power. This singleness of refraction,
however, betrays the paste imitation when it is colored to resemble
ruby, sapphire or emerald, all of which are doubly refracting.

The color is imparted to pastes by the addition, during their
manufacture, of various metallic oxides in small proportions. Thus
cobalt gives a blue color, copper or chromium green, copper or gold give
red (under proper treatment) and manganese gives purple. By experiment
the makers of pastes have become very skillful in imitating the color of
almost any precious stone. Fine paste emeralds may look better than
inferior genuine emeralds.

As pastes are singly refracting and hence lack dichroism, the pleasing
variety of color of the true ruby cannot be had in a paste imitation,
but the public is not critical enough to notice this lack. The expert
would, however, note it and could detect the imitation by that
difference as well as by the lack of double refraction. The use of
direct sunlight and a white card as already explained in the lesson on
double refraction (Lesson III.) will serve to expose the singleness of
refraction of paste imitations. Spinels and garnets are about the only
true gems (except diamond) that are single refracting. Any other color
stone should show double refraction when tested by the sunlight-card
method. The file test will also expose any paste imitation as a