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PRECIOUS STONES.
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213
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hence
known as labradorescence. It is found in augite-syenite at Laurvik in
Norway, and though not much used for cutting in small pieces, it is
frequently to be seen in this syenite, slabs of which are largely used
now in facing the jambs and lintels of shops in large towns.
All
these varieties of Orthoclase may be distinguished from similar
varieties of triclinic Felspars by the absence of the characteristic
striping of these latter species.
315. MlCROCLINE.
This
Felspar occurs white, cream coloured, red, and green: the only kind
with which we are concerned is the green, which is known as
Amazonstone. The exact colour varies somewhat, but may usually be
described as a verdigris green ; it is sub-translucent to opaque. The
specific gravity is 2-54 to 2-57. It has a
vitreous lustre, uneven fracture, and the hardness is 6 to 6-1/2.
Besides the cleavages all Felspars show it has two other less perfect
cleavages. It often shows a characteristic corded structure, and it
crystalÂlises in triclinic forms. Its mode of occurrence and
composition are very similar to Orthoclase, but the colour is supposed
to be due to traces of copper compounds. In the Ural Mountains it is
found near Minsk; in North America, at Pike's Peak in Colorado, and
with other FelÂspars at Amelia Court House in Virginia. Fine crystals
were obtained by Professor Heddle near Tongue in Scotland. It is cut
for small ornamental objects and for brooches, rings, and the like. In
the latter cases the usual form given is that of a thin plate either
flat or curved.
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