Corundum is the mineralogical or gemological name for aluminium oxide. It is better known
for the two closely related varieties of gemstone, sapphire and ruby . . . but it comes in
other colors, too.
Colors
Corundum can be colorless, red, pink, blue, black, brown, orange, yellow, green, indigo,
violet, or mauve. Red corundum is called ruby, blue corundum is called sapphire, and other
colors are also called sapphire, usually with the color specified as a prefix to the word
sapphire, for example, yellow sapphire.
Coloration
Pure corundum is colorless, often called white, and although quite rare, tends not to be
valuable because it does not have much brilliance. Colors, as in many gemstones, are caused
by small amounts of impurity, usually metallic oxides. This is a case where impurity is
desirable.
Chromic oxide causes brilliant red coloring in corundum, thereby producing rubies.
Ferric oxide causes yellow coloration, titanium oxide produces vivid blue. In fact the
coloration of corundum is not quite so simple as this. The titanium and iron are usually
present in the form of ilmenite, a mineral that is a titanium iron oxide, TiFeO3. Ilmenite
is not isomorphous with aluminium oxide. Isomorphous means being able to replace the host
mineral within its crystal structure. Instead ilmenite is present as a microscopic inclusion,
in the form of colloidal particles.
The colloidal nature of corundum may be responsible for other optical effects such as
"silk", asterism, and color banding.
Durability
Corundum is very hard, having a hardness of 9 on Moh's scale, compared with 10 for diamond,
and 8 for topaz. Hardness is generally a desirable feature is gemstones.
Other uses for corundum, because of its hardness, are as watch bearings, watch glasses,
and as an abrasive.
Technical Characteristics
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Chemical Composition:
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Al2O3 - Aluminium Oxide
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Hardness:
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9
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Specific Gravity:
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3.96 - 4.01
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Refractive Index (R.I.):
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1.759 - 1.767 to 1.770 - 1.779
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Bi-refringence:
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0.009
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Optic Sign:
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Positive
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Crystal Structure:
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Trigonal
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