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Corundum |
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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 AbrasiveCorundum 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. EmeryEmery 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: RubyMost 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: SapphireCorundum 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 SapphiresMost 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 OccurrenceCorundum 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 CorundumsNatural 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. |
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 Red corundum (ruby) in zoisite. (More detailed images below.)
 Deep Blue Corundums are "Sapphires" © iStockphoto / mirajewel  Emery rock, rich in corundum and spinel. (More detailed images below.)  Deep Red Corundums are "Rubies" © iStockphoto / Wilson Valentin  Corundum in feldspar. (More detailed images below.)  Colorful sapphires from Sri Lanka (More detailed images below.) |
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 from Merkestein, Transvaal, South Africa. Specimen is about six inches (fifteen centimeters) across. 
Red corundum (ruby) from Madagascar. Each specimen is about 1/2 inch (13 millimeters across).  Colorful corundums (sapphires) from Sri Lanka. Specimens are about 1/4 inch (six millimeters) across 
Deep blue corundum (sapphire) from New South Wales, Australia. Specimens are about 1/4 inch (six millimeters) across 
Emery rock rich in corundum and spinel from Peekskill, New York. This specimen is approximately six inches (fifteen centimeters) across. 
Corundum in feldspar from near Ontario, Canada. Specimen is about six inches (fifteen centimeters) across.
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