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PRECIOUS STONES.
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there
are three other less important cleavage planes; the lustre of the more
perfect cleavage plane is distinctly pearly. From the less perfect
cleavage, on turning in certain directions most beautiful colours will
be suddenly seen proceeding when the angle the line of sight makes with
the cleavage plane is exactly right; in all other positions the mineral
is but a dull grey colour. This effect, from being seen in this
mineral, is known as labradores-cence ; the colours are very
varied—green, yellow, blue, or red, and these in various shades may be
seen. The colour in some cases is due to minute inclusions causing
interference of light; but in the case of the blue colour it must be
due to some other cause of interference, since it may be seen when
there are no inclusions present.
The
specific gravity of Labradorite is 2.67 to 2'76; its hardness is rather
below the average of the Felspars, being only 5 to 6. In crystalline
form it is triclinic, and it often shows repeated twinning, and this
may cause the flash of light to be broken up by rectangular patches of
dull grey, representing a portion of the crystal occupying a reversed
position; when the specimen is turned the parts which were previously
grey may be seen to light up with colour and at the same time the
previously brilliant portions will be dull.
In
chemical composition it is an aluminium-calcium-sodium silicate ; it is
a common constituent of basic eruptive rocks. It was first found in
the island of St. Paul, off the east coast of Labrador; on the mainland
of Labrador several localities near Nain are given. In Russia it was
found in boulders at Peterhof, and in some of the cobbles used in
paving the St. Petersburg streets. Other localities
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