Having now an indication
as to the hardness of the unknown gem look up in the table of the
previous lesson those gems of similar hardness and then by the use of
some of the tests already given decide which of the stones of that
degree of hardness you have. _Never rely upon a single test in
identifying a gem._

For further study of hardness and its use in testing gems see
_Gem-Stones_, G. F. Herbert-Smith, Chap. IX., pp. 78-81, and table on p.
305; or see _A Handbook of Precious Stones_, Rothschild, pp. 19, 20,
21.

LESSON X

DISPERSION

Another property which may be made use of in deciding the identity of
certain gems is that called _dispersion_. We have seen in Lesson II.
that light in entering a stone from the air changes its path
(refraction), and in Lesson III. it was explained that many minerals
cause light that enters them, to divide and proceed along two different
paths (double refraction). Now it is further true that light of the
various colors (red, orange, yellow, green, blue, and violet) is
refracted variously--the violet being bent most sharply, the red least,
and the other colors to intermediate degrees. The cut (Fig. 7)
represents roughly and in an exaggerated manner the effect we are
discussing.

[Illustration: FIG. 7.]

Now in a cut stone this separation of light of different colors, or
dispersion of light, as it is called, results in the reflection of each
of the colors separately from the steep sloping back facets of the
stone. If almost any clear, colorless facetted stone is placed in the
sunlight and a card held before it to receive the reflections, it will
be seen that rainbow-like reflections appear on the card. These
_spectra_, as they are called, are caused by the dispersion of light.
With a diamond the spectra will be very brilliant and of vivid coloring,
and the red will be widely separated from the blue. With white sapphire
or white topaz, or with rock crystal (quartz), the spectra will be less
vivid--they will appear in pairs (due to the double refraction of these
minerals), and the red and blue will be near together (_i. e._, the
spectra will be short). This shortness in the latter cases is due to the
small dispersive power of the three minerals mentioned. Paste (lead
glass) gives fairly vivid spectra, and they are single like those from
diamond, as glass is singly refracting. The dispersion of the heavy lead
glass approaches that of diamond. The decolorized zircon (jargoon)