has a
dispersion well up toward that of diamond and gives fairly vivid spectra
on a card, but they are double, as zircon is doubly refracting. Sphene
(a gem rarely seen in the trade) and the demantoid garnet (a green gem
often called "olivine" in the trade) both have very high dispersive
power, exceeding the diamond in this respect. As they are both colored
stones (sphene is usually yellowish, sometimes greenish or brown), the
vividness of their color-play is much diminished by absorption of light
within them. So also the color-play of a deeply colored fancy diamond
is diminished by absorption.

DISPERSION AS A TEST OF THE IDENTITY OF A GEM. We may now consider how
an acquaintance with the dispersive powers of the various stones can be
used in distinguishing them. If a stone has high dispersive power it
will exhibit "fire," as it is called--_i. e._, the various colors will
be so widely separated within the stone, and hence reflected out so
widely separated, that they will fall on the eye (as on the card above)
in separate layers, and vivid flashes of red or yellow or other colors
will be seen. Such stones as the white sapphire (and others of small
dispersion), however, while separating the various colors appreciably as
seen reflected on a card, do not sufficiently separate them to produce
the "fire" effect when the light falls on the eye. This is because the
various colors, being very near together in this case, cross the eye so
rapidly, when the stone is moved, that they blend their effect and the
eye regards the light that thus falls upon it as white. We have here a
ready means of distinguishing the diamond from most other colorless
gems. The trained diamond expert relies (probably unconsciously) upon
the dispersive effect (or "fire") nearly as much as upon the adamantine
luster, in telling at a glance whether a stone is or is not a diamond.
Of all colorless stones, the only one likely to mislead the expert in
this respect is the whitened zircon (jargoon), which has almost
adamantine luster and in addition nearly as high dispersive power as
diamond. However, zircon is doubly refracting (strongly so), and the
division of the spectra which results (each facet producing two instead
of only one) weakens the "fire" so that even the best zircon is a bit
"sleepy" as compared with even an ordinary diamond.

In addition to providing a ready means of identifying the diamond, a
high degree of dispersion in a stone of pronounce