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.

FIG. 7.

Now in a cut stone this separation of light[61] 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[62] 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)
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[63] 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[64] 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 pronounced color would lead one to consider sphene, demantoid garnet[65] (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.