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Corundum

An extremely hard mineral, aluminum oxide, Al₂O₃, sometimes containing iron, magnesia, or silica, that occurs in gem varieties such as ruby and sapphire and in a common black, brown, or blue form used chiefly in abrasives.

Pure corundum is transparent and colorless, but most specimens contain some transition elements substituting for aluminum, resulting in the presence of color. Substitution of chromium results in a deep red color; such red corundum is known as ruby. The term “sapphire” is used in both a restricted sense for the “cornflower blue” variety containing iron and titanium, and in a general sense for gem-quality corundums of any color other than red. Star ruby and star sapphire contain tiny needies of the mineral rutile.

See also: Ruby; Sapphire.

Corundum occurs as a rock-forming mineral in both metamorphic and igneous rocks, but only in those which are relatively poor in silica, and never in association with free silica. Igneous rocks which most commonly contain corundum include syenites, nepheline syenites, and syenite pegmatites. Both contact and regionally metamorphosed silica-poor rocks may contain corundum. See also Igneous rocks; Metamorphic rocks.

Material Notes:
Property values below are taken from literature and manufacturer sources. Variations are possible among various commercial grades.

Corundum is the mineralogical name for aluminium oxide. Corundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum and most pink corundum is called ruby, blue corundum is called sapphire, and other colours are also called sapphire, usually with the colour specified as a prefix to the word sapphire, for example, yellow sapphire.

Pure corundum is colourless, often called white, and although quite rare, tends not to be valuable because it does not have much brilliance. Colours, 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 colouring in corundum, thereby producing rubies.
Ferric oxide causes yellow colouration, titanium oxide produces vivid blue.
In fact the colouration of sapphire is not quite so simple as this. The titanium and iron are usually present in the form of ilmenite, a mineral which 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.
This colloidal nature may be responsible for other optical effects such as "silk", asterism, and colour banding.

Corundum, in the form of industrial alumina, is an important commodity. Alumina grit is the working ingredient of sandpaper, and sapphire plates and rods are used in many high-tech applications. Most corundum jewelry uses manufactured stones today.

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