With
the exception of Diamond all are softer than Corundum, and Sapphire is
the hardest form of Corundum. Without exception all those mentioned are
of lower specific gravity ; but Cyanite and Spinel might sink with
Sapphire in a saturated solution of iodine and iodoform in methylene
iodide (the heaviest solution of Max Bauer). Of these two Spinel is a
singly refracting mineral, and is not dichroic, while Cyanite is less
transparent, and often has a slightly pearly lustre.
Glass imitations, though of good colour, are singly refracting, and very much softer than Sapphire.
One
of the finest and most perfect Sapphires is the "Rospoli" gem, now in
the Museum of the Jardin des Plantes, in Paris; it weighs 132 carats. A
stone of 951 carats was in the possession of the King of Ava at one
time.
Star
Sapphire, or Asteriated Sapphire. This is a variety which shows a
six-rayed star when viewed on the basal plane, or on a facet cut
parallel to the base ; if the star is well defined, and the rays are
bright, the stone is of considerable value; sometimes, however, it
merely shows as a bright band of light, and not a complete star. The
star characteristically exhibits a shimmering silvery light. It is due
either to reflection from the twin lamellae, which give rise to the
parting planes, or to the existence of three sets of planes of minute
cavities, intersecting one another in the vertical axis of the crystal.
Star Sapphires are always cut en cabochon.
Leuco-Sapphire
is colourless crystallised Corundum. It is in some cases the result of
acting on inferior Sapphires, which are not of a colour good enough for
gem use, by
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