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| Quartz
sometimes displays a cat's-eye effect caused by fibrous
ns. Colors of such stones are generally grayish, black,
gray¬-brown, greenish-yellow, and olive-green.
The
eye effect in quartz is - than in tigereye, but such
gems are not as frequently seen in the gem trade.
Chalcedony
Quartz commonly forms as tiny grains or fibrous crystals,
which may w packed together and stained various colors
by natural pigments.
¬This
produces an enormous variety of patterns, color banding
effects and wispy inclusions in a translucent matrix.
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Such material is - as cryptocrystalline (microscopically
crystallized) quartz, and colored gem varieties are
called agate and jasper. The basic cryptocrystall¬ine
matrix is called chalcedony. |
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Chalcedony
occurs throughout the world and is one of the most m and least
expensive of all gem materials. Agate was known to e man,
and acquired a cloak of superstition. It was supposed to cure
insomnia, breed prudence and caution, and bring its owner
and victory in battle. Engraved and intricately worked chal¬cedony
tools were made thousands of years before the birth of Christ.
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Quartz
Gems
Rutilated quartz, sometimes called sagenite, contains
inclusion golden or reddish rutile. Some sagenites
contain fibrous inclusion of ¬epidote, actinolite,
or other minerals, but rutile in long needle crystals
is the most common. The finest such material comes
r, Brazil.Tourmaline in quartz takes the form
of black, rod-like crystals. Face w gems are curiosities
and highly decorative.
Dendritic
quartz is so named because of the presence of
tree-~ - inclusions. These are confined to thin
layers within the quartz a produce interesting
effects. The dendrites are actually deposits of
minerals, such as manganese oxides, that precipitated
along thin fractures within the quartz.
Aventurine is a rock made up of interlocked grains
of quartz, contain small crystals of the mineral
mica. In some varieties the mica is a green chrome-rich
mineral called fuchsite, which colors the quartz
a n, green. The mica flakes tend to be oriented
in parallel and reflection from ¬the myriad
flakes produces a sparkling sheen. Most of the
green aventurine seen in jewelry comes from India.Tiger's-eye,
or tigereye, is a popular and curious material.
It consists of ¬compact fibers of quartz that
has replaced asbestos fibers. The light reflected
from the fibers produces a strong sheen or silky
appearance,
.
‘like
that of the original asbestos, known as chatoyancy.
Sometimes - in the color density of the material
produces a "cat's-eye" effect, but more
often the appearance is that of shimmering bands
of light. tigereye may vary in color from brown
and brownish-yellow to blue and green. |
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The
latter hues are generally referred to as "v e or falcon's-eye.
Tigereye may be bleached in hydrochloric i dyed various colors,
including red, yellow, gray, blue, and green. these are attractive
colors but do not occur naturally.
Heating yellow-brown tigereye can sometimes oxidize the iron
that causes the coloration, producing reddish tones.
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