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INTRODUCTION |
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The article below is a version of a slide-illustated talk
periodically given at gem and mineral club meetings.
Most of the photos are thumbnailed, so clicking on them will
reveal the full-resolution images behind them. |
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"CORUNDUM
- A VISUAL EXPERIENCE" |
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Corundum is a special mineral, not only because of
its economic importance (in gem form it is ruby or
sapphire; and emery abrasive is a sub-gem form), but
also because of its diversity of colors, forms,
associations, and source localities which makes it
uniquely attractive to collectors. The specimens
illustrated here are selected from our
research/display collection to reflect this
diversity. |
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California, Nepal, Burma,
Russia |
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In
its pure form, corundum is a transparent, colorless
form of aluminum oxide, Al2O3.
It crystallizes in the hexagonal subsystem of
the trigonal system. Hexagonal prisms or
pyramids and modifications are characteristic forms.
It is extremely hard (9 on the Mohs scale) and
rather dense (specific gravity about 4.00). |
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Prism (Kenya) |
Prisms (Brazil) |
Pyramid (Burma) |
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Perhaps the most attractive are the bipyramids such
as the classic Sri Lankan "ruby" or the
"geuda" crystal on the left. The cluster
of bipyramidal crystals on the near right is from
the classic Kashmir alluvial deposit (note
the white kaolin attached). Pure
corundum is colorless and tansparent. The far
right stone
and the one above it are gemmy "white sapphires",
sometimes called "poor man's diamonds".
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Bipyramid (Sri
Lanka) |
Bipyramid (Sri
Lanka) |
Bipyramid (Kashmir) |
Bitapered prism
(Burma) |
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Hexagonal prismatic crystals occur in a variety of
proportions: "elongated", "prismatic" (I know,
redundant), "tabular", and "wafer", for example.
Those to the right also show some of the color
variety. They are (left to right) from "Zoutspansburg",
South Africa, India. Madagascar; Burma, and
Tanzania. The two tabular crystals on the
rught are from Montana and Burma |
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Prisms |
Montana |
Burma |
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Traces of
other metallic ("transition") elements replacing aluminum in
the crystal lattice can produce just about any color of the
rainbow.
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The
"pigeon-blood red" of the finest rubies is usually
caused by chromium. Mineralogically speaking, it
may be said that any chromium-rich corundum with
reddish color is ruby (though jewelers may tend to
draw the line with less tolerance). |
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Ugandan Road Kill Pigeon |
Star Ruby Crystal (india) |
Large gems (Burma) |
Tanzania |
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The
blue and fancy-colored sapphires owe their colors
mainly to the presence of traces of iron and
titanium. See the article on origin of color
in the "Science" section for the technicalities
(article in preparation). |
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Blue (Viet Nam) |
Green (Russia) |
Brown (South Africa) |
Purple (Pakistan) |
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Corundum can
occur as single crystals, twins, clusters, and even in
massive forms.
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Crystals can have lateral faces of different forms.
Below are examples of twins, clusters, and massive
specimens, and even a rare corundum of hydrothermal
origin!. |
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Smooth (Madagascar) |
Striated (Viet Nam) |
Layered (India) |
Different (India) |
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Interpenetrant twins (India) |
Contact twins (Afghanistan) |
Tabular cluster (Burma) |
Spicular cluster (Nepal) |
Massive (North Carolina) |
Hydrothermal (Japan) |
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There are many different rocks and minerals that may be
found with corundum in matrix specimens. Perhaps the
most common is calcite. Here are some of the other
possibilities. The last specimen shows ruby which
formed in a clay bed as it petrified - with opalized snail
shells!! |
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Calcite (Viet Nam) |
Diopside, calcite (Tanzania) |
Smaragdite (North Carolina |
Syenite (Viet Nam) |
Petrified clay (Kenya) |
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Gneiss (India) |
Gneiss (Norway) |
Zoisite (Tanzania |
Zoisite (India) |
Fuchsite (Wyoming) |
Schorl (South Carolina) |
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