yellows
and reds the result of the reflection of light from minute included
crystals of magnetite, hematite, and ilmenite.
The
gems known as moonstone and sunstone owe the play of colors which gives
them their respective names to similar causes. These gems are generally
some form of feldspar, although any mineral giving a similar sheen of
color might be included under them. The moonstone of commerce comes
chiefly from Ceylon, where it occurs in large pieces the size of a fist
in a clay resulting from the decomposition of a por-phyritic rock.
Pieces of these when polished exhibit a beautiful pale blue light
coming from within, which makes the stone prized as a gem. The cause of
this light has usually been thought to be reflection from minute
tabular crystals lying in parallel position in the stone. It seems to
be partly caused also by absorption of red and yellow rays by the
stone, leaving the violet and blue to be reflected and diffused.
Moonstone
varies from translucent to opaque, and from colorless to white, the
essential feature being the blue opalescent light or chatoyancy
exhibited from a polished surface. Good feldspar moonstones are worth
from three to five dollars a carat.
The
Ceylon moonstone is sometimes known as Ceylon opal, but it is not opal.
On the contrary, it is the variety of feldspar known as orthoclase,
which is a potash feldspar, differing from the microcline just
described in being monoclinic in crystallization and in having two
cleavages meeting at right angles. Another species of feldspar used as
moonstone is albite. This is a soda feldspar, and is triclinic, but
exhibits the chatoyancy characteristic of moonstone. One variety is
known as peristerite, from the Greek word for pigeon, and is applied on
account of the resemblance of the sheen to that of a pigeon's neck. It
is found 'at Macomb, St. Lawrence County, New York. Albite found at
Mineral Hill, Pennsylvania, also exhibits the chatoyancy of moonstone.
Amelia Court House, Virginia, is another locality whence come pieces
either of orthoclase or oligoclase exhibiting this property. Like most
of tbe more or less opaque gems, moonstone is cut chiefly in the
cabochon form. It is of late, however, cut in the form of balls, which
are quite popular, the bringing of good luck being attributed to them.
The brilliancy of moonÂstone is considerably increased by mounting it
against black.
In
the Middle Ages carrying a moonstone in the mouth was believed to be an
aid to the memory. It was prescribed in cases of epilepsy, either as an
amulet or powder. Belief in its efficacy for this purpose still
persists among the Basques. During the period of increase of the moon
it was regarded a potent love charm, while during the decrease of that
luminary it was supposed to enable its wearer to foretell the future.
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