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Corundum


Corundum is frequently used as an abrasive and is world famous as the mineral of rubies and sapphires



Made Famous by Rubies and Sapphires



Most people are familiar with corundum; however, very few people know it by its mineral name. A gemstone-quality specimen of corundum with a deep red color is known as a "ruby". A gemstone-quality corundum of with a blue color is called a "sapphire".

Rubies and sapphires are famous throughout the world, but most people do not know that they are color varieties of the same mineral, corundum.


Properties and Occurrence of Corundum



Corundum is an exceptionally hard and tough form of aluminum oxide (Al2O3). As a primary mineral it is found in igneous rocks such as syenite, nepheline syenite and pegmatite. It 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.

Corundum's toughness, high hardness and chemical resistance enable it to persist in sediments long after other minerals have been destroyed. It thereby becomes concentrated in alluvial deposits. These deposits are sources of rubies and sapphires in several parts of the world. Notable deposits of alluvial rubies and sapphires occur in: Burma, Cambodia, Sri Lanka, India, Afghanistan, Montana and other areas.


Hardness and Use as an Abrasive



Corundum is very hard. It serves as the index mineral with a hardness of nine on the Mohs Hardness Scale. It is the third hardest mineral known, with diamond and moissanite being the only minerals with a greater hardness.

This high hardness makes corundum especially useful as an abrasive. Crushed corundum is screened to produce uniformly-sized grits and powders. These are used as grinding media and used to manufacture polishing compounds, sand papers, grinding wheels and cutting tools.

The costs of manufacturing abrasives have been declining through innovation. Synthetic corundum is increasingly being used as an abrasive instead of natural corundum. Some of it is manufactured from calcined bauxite which yields Al2O3 with the same Mohs Scale 9 hardness as natural corundum.


Emery



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 used to manufacture abrasives.

The use of corundum as an abrasive has declined in the last few decades. It is being replaced by manufactured abrasives such as silicon carbide. Silicon carbide has a Mohs hardness of 9 to 9.5. It is inexpensive and often performs better than natural abrasives made from corundum or emery.


Physical Properties of Corundum

Chemical Classification oxide
Color frequently gray but also white, brown, red, blue, yellow, green
Streak colorless (harder than the streak plate)
Luster adamantine to bitreous
Diaphaneity transparent to translucent
Cleavage none However, corundum does display parting perpendicular to the long axis of its hexagonal crystals
Mohs Hardness 9
Specific Gravity 3.9 to 4.1
Diagnostic Properties hardness, high specific gravity, often found as six-sided hexagonal crystals that sometines tapers into a pyramid - often with parting, high luster, conchoidal fracture
Chemical Composition Al2O3
Crystal System hexagonal
Uses


Red Corundum: Ruby



Most corundum occurs as white, gray, dull blue or dull red crystals. However, a very small amount of corundum has a transparency, purity and color that make it suitable for use as a gemstone. These colorful corundums are among the most popular and valuable gems.

Some gem-quality corundum contains trace amounts of chromium that substitutes in the crystal for aluminum. A very small amount of chromium gives corundum a pink color. Larger amounts produce stones that are a deep traffic light red. These deep red corundums are known as "rubies". Transparent rubies of gem quality are cut into faceted stones and translucent rubies of gem quality are cut into cabochon-shaped gemstones.


Blue Corundum: Sapphire



Corundum that has small amounts of iron or titanium substituting for aluminum in the crystal structure has a blue color. Deep blue corundums are known as "sapphires". When used alone, the word "sapphire" always refers to a deep blue corundum.

Gem quality corundum occurs in a wide range of colors, including pink, pale blue, yellow and green. These stones are also known as "sapphires", however, when the color of a sapphire is any color other than deep blue the color of the stone is always used as an preceding adjective. For example: pink sapphire, yellow sapphire, green sapphire. Used alone, the word "sapphire" refers to the deep blue corundum.

The color of corundum can be artificially altered by heating or radiation. Sometimes cloudy or translucent stones can be heated to yield brighter color or more transparent stones. When these color treatments are done the stone is supposed to be labeled as "heat treated" when it is presented for sale.


Mining Rubies and Sapphires



Most gem-grade corundums form in metamorphic rocks such as schists or igneous rocks such as syenite. However, they are rarely mined from the rocks in which they form. Instead, they are usually mined from stream sediments.

Rubies and sapphires are very hard and resistant to chemical weathering. These characteristics enable them to survive the abrasive actions of a stream. Their high specific gravity, relative to other sediment particles, often causes currents to concentrate them in small deposits. Most rubies and sapphires are produced by washing the gravels of these stream deposits.

Noteworthy locations where gem-quality corundums have been produced include: Myanmar, Thailand, Cambodia, Vietnam, India, Pakistan, Afghanistan, Sri Lanka, China, Australia, Montana, Kenya, Tanzania, Nigeria and Malawi.


Synthetic Corundums



Natural deposits of corundum are often impure and not available in large quantities where it is needed for manufacturing abrasives. This need is often met by producing synthetic corundum from bauxite. The synthetic corundum is of high quality and cost-competitive with corundum from natural sources.

Synthetic rubies and sapphires are also produced. The manufacturing process can produce large, flawless single crystals which can be cut into attractive gemstones. The color in these stones can be controlled by adding small amounts of chromium, titanium or other substance.

Special optical effects such as asterism can be imparted to the stones by adding titanium or another material which crystallizes in the form of needles (such as rutile). These needles can be in alignments which produce a "star" appearance when the stones are cut and polished. These synthetic stones must be labeled when sold. It is often difficult for even an expert to distinguish them from natural stones.


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  corundum as ruby, sapphire and fancy sapphire
Gem-quality corundum is a highly-prized and valuable material. When it is bright red in color it is called "ruby". When it is deep blue it is called "sapphire". Gem-quality corundum of any other color is called "fancy sapphire". Historically, ruby and sapphire have mainly been mined in Asia. Now, many deposits of beautiful corundums are being mined in Africa, the source of all of the stones in the photo above.




corundum - ruby in zoisite
Corundum (ruby) in zoisite from Merkestein, Transvaal, South Africa. Specimen is about six inches (fifteen centimeters) across.


corundum sapphire
Deep Blue Corundums are known as "Sapphires" © iStockphoto / mirajewel


corundum emery
Emery rock rich in corundum and spinel from Peekskill, New York. This specimen is approximately six inches (fifteen centimeters) across.


ruby corundum in ring
Deep Red Corundums are known as "Rubies" © iStockphoto / Wilson Valentin


sapphires
Colorful rough corundums (sapphires) from Sri Lanka. Specimens are about 1/4 inch (six millimeters) across.


Corundum parting
Common corundum showing parting and hexagonal habit from the Zoutpansberg District, Transvaal, South Africa. Specimen is approximately two inches (5 centimeters) across.


blue star sapphire
A deep blue star sapphire 8 mm x 6 mm cabochon from Thailand. Inclusions within the stone align with the crystallographic axis to produce the star - which is only clearly visible and centered when the back of the stone is cut at 90 degrees to the C-axis of the crystal. This stone has been heat treated to darken the stone and enhance visibility of the star.


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