This is a revised
version of slide-illustrated presentions given in
the past to gem and mineral clubs around the country. It may
be copied and distributed without restriction, except we request it
is done not for profit (exception can be made for enhancing club
treasuries). It is not in PDF format, but we hope to have that
or some other printer friendly version done soon. However, on
the computer most of the photos are thumbnailed, and clicking on
them reveals higher resolution images.
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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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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 (such
as the Kenyan sapphire on the left) and pyramidal or bipyramidal
modifications (Burma, left and
Sri Lanka, right) are characteristic
forms. |
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A most
attractive bipyramid is this classic classic Sri Lankan "geuda"
crystal on the left, which has been illustrated in pen and ink drawings
in European mineralogy books. On the right are two Brazilian
hexagonal prisms and a pyramidal cluster from the original
Kashmir alluvial
deposit - note white kaolin residue still attached. |
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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; Karnataka, India;
Madagascar; Dat Taw Mine, Mogok Stone Tract, Burma; and Tanzania. |
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Some of
the finest wafer crystals come from the United States; and one of
the finest of these (left) is this 6.62 carat flawless gem from the
Vortex Mine, Yogo Gulch, Montana. The tabular ruby on the right is
from the Dat Taw Mine, Mogok Stone Tract, Burma |
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