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GEMSTONES (12)

 

Corundum


Corundum has historically been used as an abrasive, but it is most famous as the mineral of ruby and sapphire.

Corundum

Corundum:Two corundum crystal segments from India showing the mineral's hexagonal crystal habit and basal parting. These specimens are red in color and might be called "ruby corundum." Image © iStockphoto / Lissart.

What is Corundum?

Corundum is a rock-formingmineralthat is found inigneous,metamorphic, andsedimentaryrocks. It is an aluminum oxide with a chemical composition of Al2O3and a hexagonal crystal structure.

The mineral is widely known for its extremehardnessand 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 cuttinggemstones.

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 crystals

Corundum crystals:Photos of three corundum crystals. On the left is a common corundum from Transvaal, South Africa, that is about 6 centimeters in height. In the center is a gem-quality ruby corundum from Karnataka, India, that is about 1.6 centimeters in height. On the right is a blue sapphire corundum from Sri Lanka that is about two centimeters in height. All three specimens and photos by Arkenstone /www.iRocks.com.

Made Famous by Rubies and Sapphires

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 parting

Corundum parting:Hexagonal crystal segments of corundum that have been separated by parting. These specimens are about one centimeter across. USGS photo by Andrew Silver.

corundum gneiss with sapphire

Corundum gneiss with sapphire:A specimen of corundum gneiss from Gallatin Valley, Montana. This specimen is about twelve centimeters across and has a round blue sapphire crystal on the left side.

Properties of Corundum

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

Its hardness, high specific gravity, hexagonal crystals and parting are very good diagnostic properties to use in its identification. A summary of the physical properties of corundum is given in the table below.

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,

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