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PRECIOUS STONES.
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The
difference between the indices for the B and H lines of the spectrum is
'0574, which figure is therefore the coefficient of dispersion in
Diamond.
Since
the "fire" of a stone depends on the degree to which white light is
split up into its components, it at once becomes obvious that the fire
of this gem is very marked.
Diamond,
being a cubic mineral, should always show single refraction ; but this
is not always found to be the case, anomalous double refraction
frequently being present, especially in stones that are coloured, or
contain enclosures of impurities. It is found that many crystals of
Diamond show signs of great internal strain, so great at times that a
stone will sometimes fly to pieces on being handled. This suggests that
the mineral is often formed under conditions of great pressure. Max
Bauer quotes the similar instance of " Prince Bupert's Drops," small
beads of glass formed during the blowing of glass vessels, in which
great internal strain is set up on cooling, and in which, too, double
refraction is seen, due to this strain, although glass, being a colloid
body, should only show single refraction. The smoky stones from South
Africa show the phenomenon in the most marked degree. Under the
polariscope, with the Nicol's prisms crossed, a stone showing single
refraction when rotated gives a complete dark field; but specimens
having these internal strains show irregular light portions of the
field under the same conditions. Precautions must of course be taken to
ensure that light is not entirely cut off by total internal reflection,
by surrounding the stone with some highly retractile medium during
examination. Diamond occasionally shows asterism.
Phosphorescence is strongly marked in most Diamonds,
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