[ko run' dum] a transparent to
translucent mineral which is found in white,
blue, red, yellow, green, brown, purple, and pink
forms, as well as in colorless forms
Chemically, corundum is
alumina, or aluminum oxide, Al2O2. Its
distinguishing characteristic is its hardness,
which is next to that of diamond and is rated 9
Corundum crystals are six-sided
and are often found as tapering hexagonal
bipyramids, rounded into barrel shapes.
Next to silica, alumina is the
most abundant oxide in the earth's crust. It is
usually found in igneous rocks deficient in
silica. During the crystallization of magma,
alumina ordinarily would combine with silica and
other oxides to form feldspars. If, however, the
magma lacks silica, corundum crystallizes out
during solidification. There are notable deposits
in Myanmar, Sri Lanka, and India, as well as in
several African localities, Middle Eastern and
Southeast Asian countries, and in North Carolina
and Montana in the United States.
The various colors of corundum
are due to impurities. When chromium oxide is
present, corundum is red and is known as ruby.
The deep red ruby is a very valuable gem stone.
Other impurities result in a blue color, and the
mineral is called the sapphire. Varieties which
have a stellate opalescence when observed in the
direction of one of the crystal axes are called
asteriated and are more popularly known as star
sapphires or star rubies.
Corundum was once extensively
used as an abrasive because of its hardness, but
has been replaced in many applications by
artificial corundum made from bauxite at high
temperatures. Emery is a fine-grained mixture of
corundum and magnetite. It is still used in the
manufacture of some grinding wheels.
The Mohs Hardness Scale
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