Corundum

Corundum is a rock-forming mineral that is found in igneous, metamorphic, and sedimentary rocks. It is an aluminum oxide with a chemical composition of Al2O3 and a hexagonal crystal structure.
The mineral is widely known for its extreme hardness and for the fact that it is sometimes found as beautiful transparent crystals in many different colors. The extreme hardness makes corundum an excellent abrasive, and when that hardness is found in beautiful crystals, you have the perfect material for cutting gemstones.

Natural and synthetic corundum are used in a wide variety of industrial applications because of their toughness, hardness, and chemical stability. They are used to make industrial bearings, scratch-resistant windows for electronic instruments, wafers for circuit boards, and many other products.

Corundum

Most people are familiar with corundum; however, very few people know it by its mineral name - instead they know it by the names "ruby" and "sapphire." A gemstone-quality specimen of corundum with a deep red color is known as a "ruby." A gemstone-quality corundum with a blue color is called a "sapphire." Colorless corundum is known as "white sapphire." Corundum of any other color is known as "fancy sapphire."

Corundum Crystals

Geologic Occurrence

Corundum is found as a primary mineral in igneous rocks such as syenite, nepheline syenite, and pegmatite. Some of the world's most important ruby and sapphire deposits are found where the gems have weathered from basalt flows and are now found in the downslope soils and sediments.
Corundum is also found in metamorphic rocks in locations where aluminous shales or bauxites have been exposed to contact metamorphism. Schist, gneiss, and marble produced by regional metamorphism will sometimes contain corundum. Some of the sapphires and rubies of highest quality, color, and clarity are formed in marble along the edges of subsurface magma bodies.

Corundum's toughness, high hardness, and chemical resistance enable it to persist in sediments long after other minerals have been destroyed. This is why it is often found concentrated in alluvial deposits. These deposits are the most important source of rubies and sapphires in several parts of the world. Traditional sources of alluvial rubies and sapphires include Burma, Cambodia, Sri Lanka, India, Afghanistan, Montana, and other areas. In the past few decades, several parts of Africa, including Madagascar, Kenya, Tanzania, Nigeria, and Malawi, have become important producers of ruby and sapphire.

Properties of Corundum

Corundum is an exceptionally hard and tough material. It is the third-hardest mineral, after diamond and moissanite. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale.


Physical Properties of Corundum

Chemical Classification Oxide
Color Typically gray to brown. Colorless when pure, but trace amounts of various metals produce almost any color. Chromium produces red (ruby) and combinations of iron and titanium produce blue (sapphire).
Streak Colorless (harder than the streak plate)
Luster Adamantine to vitreous
Diaphaneity Transparent to translucent
Cleavage None. Corundum does display parting perpendicular to the c-axis.
Mohs Hardness 9
Specific Gravity 3.9 to 4.1 (very high for a nonmetallic mineral)
Diagnostic Properties Hardness, high specific gravity, hexagonal crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture
Chemical Composition Al2O3
Crystal System Hexagonal
Uses Historically used as an abrasive. Specimens with pleasing colors have a long history of gemstone use.

Its hardness, high specific gravity, hexagonal crystals and parting are very good diagnostic properties to use in its identification.

Uses

The extreme hardness of corundum makes it especially useful as an abrasive. Crushed corundum is processed to remove impurities and then screened to produce uniformly sized granules and powders. These are used for grinding media, polishing compounds, sand papers, grinding wheels, and other cutting applications.

Emery stone is a granular metamorphic or igneous rock that is rich in corundum. It is a mixture of oxide minerals, typically corundum, magnetite, spinel and/or hematite. It is the most common form of natural corundum that has been used to manufacture abrasives.

In the gemstone and jewelry market, almost all of the attention goes to a small group of gems known as "the big four": diamond, ruby, sapphire, and emerald. Two of these, ruby and sapphire, are gem corundums.



Sources: Geology.com

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