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Evaluating
Gem Diamonds
Australia
is known for its production of very dark blue and blue
green sapphires, especially from Anakie, Queensland.
Corundum gems of other colors have also been mined here.
Gem corundum occurs in the United States in North Carolina
and Montana. The mines at Macon County, North Carolina
are open to the public on a fee basis, but no early
fine gems occur here.
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Near
Helena, Montana, sapphires occur in alluvial deposits
and were recovered in gold-mining operations. The best
known corundum deposit in North America is at Yogo Gulch
in Fergus County, Montana. Here sapphires are found
that yield fine blue gems up to 3-4 carats in weight,
but most of the stones found are smaller. The crystals
from Yogo Gulch tend to be flattened and plate like,
whereas corundum usually occurs in barrel-shaped crystals. |
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Corundum
crystallizes in six-sided forms, as dictated by the internal
atomic arrangement of the mineral. When corundum crystals
form, they occasionally incorporate inclusions of other minerals,
one of which is Val utile (titanium oxide). Retile tends to
occur in elongated or fibrous crystals; such crystals within
corundum orient themselves according to the six-fold symmetry
of the corundum host material. Light is reflected 'room the
rutile needles, producing a diffuse "sheen." When
a cabochon , cut from such material, the reflections are sharpened
along the curved nippers surface of the stone.
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DIAMONDS,
GEMSTONES & CRYSTALS
FACTS & FIGURES |
| Evaluating
Gem Diamonds
This
is the phenomenon of asterism, and the six-fold
symmetry of corundum produces a six-rayed starStar
rubies and sapphires are rare gems, but they can
be large and spectacular. The finest star ruby
known weighs 138 carats; this is the famous Rosser
Reeves star ruby, on display at the Smithsonian
Institu¬tion in Washington, D.C. The world's
largest blue star sapphire, the Star of India,
exhibited at the American Museum of Natural History
in New York, weighs 563 carats.
Star
corundum of lesser size is available in the gem
trade in a wide variety of colors and degrees
of transparency. Star gems are never completely
transparent, because the star can only be produced
by the presence of needle-like inclusions that
cause some degree of turbidity. Each set of oriented
inclusions produces one ray of the star. Only
a properly cut gem, however; will display a perfectly
centered, six-rayed star. This must be cut so
that the base of the stone is perpendicular to
the direction of six-fold symmetry of the corundum
crystal.
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In
hardness corundum is second only to diamond, and
rates 9 on the Mohs scale. Cut gems are both hard
and tough, and wear extremely well in all kinds
of jewelry. |
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Ruby
and emerald are the most valuable of all gems. A large, essentially
flawless, deep-red ruby weighing more than ten to 15 carats
might cost more than $30,000 per carat. Small gems of lesser
color might cost as little as $100 per carat. Sapphires are
much less expensive, ranging in price from a few dollars per
carat to several thousand dollars per carat. Colors other
than blue are not very expensive; attractive star gems in
shades of gray or black might sell for $10 to $50 per carat. |
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