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212 PRECIOUS STONES.
Oligoclase, may occur as Moonstone. The most important locality tor this gem variety is
to the south-east of Adam's Peak in Ceylon, where Moonstone occurs in
fragments imbedded in a white clay, probably derived from the
weathering of some acid volcanic rock. Another place where it is found
is the Amelia Court House in Virginia in the United States; here it
occurs in situ in the granite.
It is always cut either in flat pieces, when it shows a uniform adularescence, or en cabochon, when
the light takes the form of a more or less well marked band. The
exposed surface must be properly placed to show the full effect. Brand
records that a good Moonstone of about half an inch in diameter sold
for £35. It is readily distinguished from a glass imitation by the double refraction of the Felspar, and by the glass being more dense.
Earely
Orthoclase occurs in the form of Aventurine, verisimilar in appearance
to the Aventurine variety of Quartz. The effect is due to the inclusion
along the directions of the more perfect cleavage plane of numerous
plates of micaceous Haematite which cause a bright red metallic
reflection of the light. Aventurine Felspar is also known as Sunstone,
and here again the term applies to appearance rather than to
composition. Orthoclase Sunstone is not so common as the Oligoclase
Sunstone. The Orthoclase variety is found chiefly in North America, at
Amelia Court House in Virginia and in New York State. It is cut en cabochon, and the surface must of course be in such a position that the reflection is seen at its best.
A rare variety of Orthoclase is that which shows an optical colour effect similar to that seen in Labradorite, and
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