d color would lead one
to consider sphene, demantoid garnet (if green), and zircon (which
might be reddish, yellowish, brown, or of other colors), and if the
stone did not agree with these in its other properties one should
suspect _glass_.

A good way to note the degree of dispersion, aside from the
sunlight-card method, is to look at the stone from the back while
holding it up to the light (daylight). Stones of high dispersive power
will display vivid color play in this position. Glass imitations of
rubies, emeralds, amethysts, etc., will display altogether too much
dispersion for the natural gems.

In Chap. III., p. 20, of G. F. Herbert-Smith's _Gem-Stones_, a brief
account of dispersion is given. College text-books on physics also treat
of it, and the latter give an account of how dispersion is measured and
what is meant by a coefficient of dispersion. Most gem books say little
about it, but as we have seen above, a knowledge of the matter can, when
supplemented by other tests, be applied practically in distinguishing
gems.

LESSON XI

COLOR

In reserving to the last the property of _color_, which many dealers in
gems use first when attempting to identify a precious stone, I have
sought to point out the fact that a determination based solely upon
color is very likely to be wrong. So many mineral species are found in
so many different colors that to attempt to identify any mineral species
by color alone is usually to invite disaster. The emerald, alone among
gems, has, when of fine color, a hue that is not approached by any other
species. The color of the grass in the springtime fitly describes it.
Yet even here the art of man has so closely counterfeited in glass the
green of the emerald that one cannot be sure of his stone by color
alone. As was suggested earlier in these lessons, the writer has
several times recently had occasion to condemn as glass imitations
stones for which high prices had been paid as genuine emeralds, those
who sold them having relied solely upon a trained eye for color.

CONFUSION OF GEMS DUE TO SIMILARITY OF COLOR. The same tendency to rely
upon color causes many in the trade to call all yellow stones "topaz"
whether the species be corundum (oriental topaz), true topaz (precious
topaz), citrine quartz (quartz topaz), heliodor (yellow beryl), jacinth
(yellow zircon), or what not.

Similarly the public calls all red stones ruby. Thus we have "cape ruby"
and "Arizona ruby" (p