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Corundum


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


What is 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 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.


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".


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.

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, 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.


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.


Geologic Occurrence of Corundum



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 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.


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.


Hardness and Use as an Abrasive



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.

One problem with using natural corundum as an abrasive is that the deposits are usually small, irregular in shape and the corundum is of variable quality. They are not reliable sources of consistant quality material needed to run a manufacturing process. Synthetic corundum, produced using calcined bauxite, has become a more reliable source with more consistent properties. It has replaced natural corundum in most manufactured products.


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 that has been used to manufacture abrasives.

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


Use as a Gemstone



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.

These most popular gems are highly sought after and have been mined in many parts of the world for thousands of years. Today, millions of rubies and sapphires are required every year to meet the demands of the jewelry market -- from inexpensive commercial stones sold in malls and department stores to spectacular specimens used in designer and custom jewelry. The demand for attractive stones exceeds the abilities of mines to supply. As a result, the prices paid for attractive natural stones have risen to high levels.

When a consumer wants a "ruby ring" or a "sapphire pendant" they are generally not interested in substituting a red spinel, blue iolite or other attractive gems of similar color. They want "ruby" or they want "sapphire". Retail jewelers, especially those selling pieces and sets for under $500, have been increasingly presenting synthetic or "lab-created" gems along-side the natural stones in their display cases.

The synthetic materials are genuine corundums. They have the same aluminum oxide composition and crystal structure as natural rubies and sapphires. Their color is also produced by the same trace elements (chromium for ruby and iron with titanium for sapphire).

They have the same optical appeal and a better physical appearance than similar-size natural stones of the same price. As a result, many consumers now gladly purchase synthetic stones because they receive a more attractive product at a price that they can afford. Over the long-term, synthetic gems are likely to continue displacing natural stones from the market, especially in the lower and middle price ranges where consumers are very conscious about price.


Corundum "Jewels" in Watches



In the mid-1800s watch makers in Switzerland needed tiny bearings that were highly resistant to abrasion. They discovered that they could drill a hole into a tiny piece of corundum and use it for a smooth-running, long-life bearing. The corundum was much harder than the metals used to make the moving parts of a watch and it was able to stand-up to the continuous abrasion without failing. This made Swiss watches and their "jewel movements" famous throughout the world for their long life and reliability. In the early 1900s, synthetic corundum bearings replaced natural corundum bearings in most Swiss watches. This use of jewel bearings created a positive reputation for Swiss watches that continues to this day - even while mechanical watches are being replaced by digital watches.


Ruby Lasers



Synthetic corundum is an essential part of many lasers. In fact, the first working laser was a "ruby laser", made by Theodore Maiman at Hughes Research Labs in 1960. It employed a synthetic ruby crystal as the "gain medium". The gain medium is a material in the laser that is the target of an intense burst of light. That light causes electrons in the gain medium to jump up to a higher energy level causing the emission of photons, which strike other atoms in the gain medium, causing them to be excited and emit more photons. This brief chain reaction produces the very intense light of a laser beam. Lasers are named after the material used as a gain medium, such as "ruby laser" or "titanium sapphire laser" or "YAG laser" (yttrium aluminum garnet).

ruby laser
Diagram of the first working laser. It employed a thin ruby crystal as its gain medium. Public domain image by Lawrence Livermore National Laboratory.

In just a few decades lasers have become common items of our society. Tiny lasers are used in CD and DVD players. Lasers are used to cut metal, stone and other tough materials. Lasers are used to remove tatoos, perform cosmetic surgery, cataract surgery and lasik surgery for vision correction.


Other Uses of Corundum



Corundum has many other uses. It is chemically inert and resistant to heat. These properties make it a perfect material for making refractory products such as fire brick, kiln liners and kiln furniture. Today, these products are usually made with synthetic corundum.

Pure corundum is colorless, transparent, durable and scratch resistant. Large crystals of clear synthetic corundum are grown, sawn into thin sheets and then used as the windows of grocery store scanners, watch crystals, aircraft windows and protective covers for electronic devices.


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  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 and Lissart.


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 left is a blue sapphire corundum from Sri Lanka that is about two centimeters in height. All three photos by Rob Lavinsky of iRocks.com used here under a Creative Commons License.


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alluvial sapphre crystals found in montana
A scatter of small alluvial sapphires found in Montana. These blue stones are untreated and measure about four to five millimeters across.

aluminum oxide sandpaper
Aluminum oxide sandpaper is made by attaching size-graded particles of synthetic corundum (aluminum oxide) to a sheet of paper. It is a sandpaper widely used for woodworking and other manufacturing work. Photo © iStockphoto and Ma-Ke.

emery wheels
An ad offering emery and corundum wheels published in 1895 by The Springfield Manufacturing Company of Bridgeport, Connecticuit. This was at a time when genuine emery and corundum were used to make the wheels.

spinel corundum emery
A specimen of emery rock that is rich in corundum and spinel from Peekskill, New York. This specimen is approximately six inches (fifteen centimeters) across. Emery has often been crushed, processed and screened for use as an industrial abrasive.
emery boards
"Emery boards" are a manicure and nail-care product that is made by gluing abrasive papers to a thin piece of cardboard. They obtained their name in the 1800s when crushed emery was used as the abrasive. Today's emery boards are not made with emery. Instead many of them have a coarse side of synthetic corundum (aluminum oxide) and a fine side of garnet abrasive. Photo © iStockphoto and Acerebel.

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 blue it is called "sapphire". When colorless it is called a "white sapphire". Gem-quality corundum of any other color is called "fancy sapphire". In the past, most gem corundum was produced in Asia and Australia. In the 1990s many gem corundum discoveries were made in Africa. All of the stones in this photo were mined in Africa. Nearly all gem corundums are treated by heating or another process to improve their color.

corundum bearings
Corundum (ruby) bearings in an antique pocket watch with a "jewel" movement. In the early 1900's synthetic corundum was being used as the jewel bearings in watches. Image iStockphoto and RobertKacpura.

corundum bearings
A drawing of jewel bearings and a capstone (red) holding a pivot wheel in a mechanical watch lubricated by oil (yellow). Publc domain image by Chris Burks Chetorno.

synthetic corundum
A boule of synthetic corundum. Because of its red color, it could be called "synthetic ruby". Material like this is used for watch bearings, gemstones, laser gain mediums and many other purposes.

scanner windows
A self check-out machine with a barcode scanner window at a retail store in Houston, Texas. The window of the scanner is probably made from synthetic corundum. Public domain image by WhisperToMe.

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