some
stones exhibiting no change whatever after exposure to sunlight.
Phosphorescence may also be called out in the diamond by rubbing it,
especially across the fibers of a piece of wood. Among all minerals
phosphorescence is best exhibited by fluorite, nearly all specimens of
which will, when gently heated, emit a visible light. The color of the
light varies with different varieties, and is usually not the same as
the natural color of the mineral. The tints exhibited are usually
greenish, bluish, or purplish. On increased heating the phosphorescence
disappears, and cannot be restored again except by passing an electric
discharge through the mineral, whereupon the lost power is usually
regained. The same is true of diamond. It is generally supposed that
the phosphorescence of minerals results from the presence within them
of particles of organic matter of the nature of hydrocarbons, which are
aroused to a certain activity on heating. Of the exact nature of the
phenomenon, however, little further is known.
Closely
allied to phosphorescence is fluorescence, which, in a strict sense, is
the emission of light within a substance while it is being exposed to
light, or in some cases to an electrical discharge from a vacuum tube.
Fluorite is again the mineral which best exhibits this property, a beam
of white light passing through a colorless cube of it producing a
delicate violet color. The diamond, ruby, and other gems are stated by
Dana to give forth a brilliant fluorescence when exposed to an
electrical discharge from the negative pole of a vacuum tube.
Fluorescence is also produced in the diamond by radio-active
substances; that is, by radium, or substances possessing its activity.
In this respect diamond differs from such gem minerals as ruby,
emerald, topaz, etc., and from glass, in none of which do the radium
rays excite much activity.
The
behavior of gems toward X-rays or Rontgen rays varies with different
species, and affords a means of distinguishing them. Thus diamond is
quite transparent to the rays, while glass is opaque. Accordingly, in
an X-ray photograph, such as is shown in the accompanying figure, of
two rings, the one set with diamonds, the other with paste, the diamond
can readily be known by its transparency.
The behavior of others of the gem minerals in relation to the X-rays is further shown in the following table:
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