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PRECIOUS STONES.
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145
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yields
lilac specimens. In North Wales, at Moel-y-Oria, dark amethyst-violet
crystals occur. At Gourock, in Scotland, both purple and green. In
Ireland, from Slieve Came in Antrim, of a green colour. Yellow crystals
of various shades are found at Gersdorf and Freiburg, in Saxony. The
rare rose-red and pink crystals are found in the Alps (St. Gothard);
flesh-red crystals at Munsterthal, in Baden. Green specimens are
commoner; Petersburg, near Halle, and Schlackenwald may be cited; also
Macomb, in New York State. Blue crystals are found in the salt-mines in
the Tyrol, and the tin-mines of the Erzgebirge.
Of
its industrial applications, besides its use as. a flux in smelting,
the manufacture of hydrofluoric acid is dependent on Fluor Spar. It has
also been used in the manufacture of apochromatic lenses.
Its
use in jewellery is largely in imitation of rarer minerals, which it
resembles in colour, and when used for this purpose it is cut after
the style that is usually adopted with the gem imitated. Such cut
specimens are styled " False Topaz," "False Sapphire," etc. Fluorescent
crystals are sometimes mounted openly (" a jour") to display their
varying colour. It is a mineral capable of receiving a high polish, but
it needs great care in handling. Vases made of the variety " Blue John"
can be turned in a lathe until very thin by first treating the material
with resin to make it more durable; when so thinly cut the colour is
well seen. The purple and red shades are often produced artificially by
heating the blue and violet kinds; it has been suggested that the
colour is here due to the presence of two hydrocarbons of different
degrees of volatility, that which causes the blue shade volatilising at
a lower
p.s.
L
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