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Corundum
Mineral Uses & Properties


Corundum is a very interesting mineral. It is an exceptionally hard aluminum oxide that forms in igneous and metamorphic rocks. It is primarily used as an abrasive. Corundum is also one of the most popular minerals - rubies and sapphires are colored corundums. Rubies are deep red corundums of gemstone quality. Gemstone quality corundums of any other color are sapphires.


Hardness and Use as an Abrasive



Corundum is very hard. It is the standard mineral for a Mohs Scale hardness of nine. It is the second hardest mineral known, with diamond being the only harder mineral.

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


Emery



Emery 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.


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 is contaminated with trace amounts of chromium. A very small amount of chromium gives corundum a pink color and larger amounts produces stones that are deep red. These deep red corundums are known as "rubies". Transparent rubies are cut into faceted stones and translucent rubies are cut into cabochon-shaped gemstones.


Blue Corundum: Sapphire



Corundum that is contaminated with iron or titanium 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 that enables 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.


Corundum Occurrence



Corundum is found in igneous rocks such as syenite, nepheline syenite and pegmatite. It is found in the rocks of contact metamorphism and in mica schist, gneiss and marble. Because it is very hard and chemically resistant, corundum is also found in alluvial deposits.


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 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. The synthetic stones are to be labeled when sold and it is often difficult for even an expert to distinguish them from natural stones.


   
corundum ruby in zoisite
Red corundum (ruby) in zoisite.
(More detailed images below.)


Corundum Properties



Corundum Photos





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


corundum emery
Emery rock, rich in corundum and spinel.
(More detailed images below.)


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


corundum in feldspar
Corundum in feldspar.
(More detailed images below.)


sapphires
Colorful sapphires from Sri Lanka
(More detailed images below.)

Corundum - Mineral Properties and Uses

Uses

Corundum is very hard and is used as an abrasive in the manufacture of sandpaper, polishing compounds and cutting tools. When it occurs as a colorful and clear stone it is used as gem. Rubies are red corundums and corundums of any other color are known as sapphires.

Color

Most often gray, but also white, brown, red, blue, yellow, green.

Streak

Corundum is harder than the streak plate. It does not leave a streak.

Luster

Adamantine to vitreous.

Diaphaneity

Transparent to translucent.

Cleavage

None. However, corundum does display parting perpendicular to the long axis of its hexagonal crystals (see photo below).

Hardness

9

Specific Gravity

3.9 - 4.1

Distinguishing
Characteristics

Hardness, specific gravity, hexagonal crystals with parting, high luster.

Crystal System

Hexagonal

Chemical Classification

Oxide

Chemical Composition

Aluminum Oxide, Al2O3


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


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


corundum ruby rough
Red corundum (ruby) from Madagascar. Each specimen is about 1/2 inch (13 millimeters across).


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


blue corundum sapphire
Deep blue corundum (sapphire) from New South Wales, Australia. Specimens are about 1/4 inch (six millimeters) across


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


corundum feldspars
Corundum in feldspar from near Ontario, Canada. Specimen is about six inches (fifteen centimeters) across.




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