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Feldspars
Moonstone,
with its mysterious white or blue sheen, is one of the
most familiar gem varieties of a large group of minerals
called the Feldspars are the most abundant minerals
in the crust of the All are aluminum silicates containing
calcium, sodium, or potassium.
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The
potassium members include orthoclase and microcline.
The other feldspars, forming the plagioclase group,
have been classified according to the relative amount
of calcium and sodium present. There are minerals in
this series which are, in sequence of increasing calcium
content albite, oligoclase, andesine, labradorite, bytownite,
anorthite. |
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Most
of the feldspars have been Cut as gems. Nearly all the plagioclases
occasionally form transparent crystals that are faceted, but
- are rare collector stones. The important feldspar gems tend
to be translucent to nearly opaque and show interesting optical
effects. Ai feldspars have a Mohs hardness of 6-61/2, which
is relatively low gem. In addition, feldspars have good cleavages,
so cut gems must worn with some care.
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Feldspars |
| Moonstone
is the most popular and important microcline gem.
Its h or white sheen is called adularescence,
and is caused by the presence a tiny crystals
of albite arranged in layers within the host microcline
Moonstone may be completely transparent, and the
sheen can be either silvery white or a soft but
distinct blue, the latter being highly prized
The adularescence sheen resembles a cloud of light
that appears with the gem when it is turned at
the right angle to the eye.
Some
moonstones are nearly opaque and may have a strong
body color, which may be beige, pink, green, yellow,
white, gray, or brown These are usually cut as
high-domed cabochons, and the sheen concentrated
at the top in a bright spot that sometimes extends
across ¬the stone as a distinct "eye."
Such gems have been called -cat's-eye moonstones."
Some others may have a second ray at right angles
to the first, forming a cross. Moonstones with
a strong body color are primarily from India.
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The
most important historical sources for fine moonstone
are Sri Lanka and upper Burma. Other sources include
Canada, Switzer¬land, Brazil, Australia, California,
Virginia, and Colorado. Mining is usually carried
out by hand, and stones are frequently cut locally.
Moonstone is still considered a sacred gem in
India and it has been used in jewelry there for
centuries. |
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Orthoclase
makes a very attractive faceted gem, especially material with
a rich golden-yellow color. The finest such material comes
from the Malaga say Republic, sometimes in huge crystals.
Cut stones weighing several hundred carats are exhibited in
major museums.
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