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 |
| Color |
Frequently gray, but also white, brown, red, blue, yellow, green. |
| Streak |
Corundum is harder than the streak plate. When it is scratched across a streak plate a white powder is produced - powdered streak plate. |
| 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 in right column). |
| Mohs Hardness |
9 |
| Specific Gravity |
3.9 - 4.1 |
| Distinguishing Characteristics |
Hardness, high specific gravity, often found as six-sided hexagonal crystals that sometimes tapers into a pyramid - often with parting, high luster. Conchoidal fracture. |
| Chemical Composition |
Al2O3 |
| Crystal System |
Hexagonal |
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.
Contributor: Hobart King
Find it on Geology.com
More from Geology.com
 | Sunstone: A feldspar with aventurescence caused by light reflecting from platy inclusions. |
| Rocks: Photos of igneous, sedimentary and metamorphic rocks. |
|
|
 | Minerals: Information about ore minerals, gem materials and rock-forming minerals. |
 | Satellite Images of cities, states and countries acquired by the Landsat satellite. |
|
|
|
| Although most people imagine a blue stone when they hear the word "sapphire", a sapphire is really a corundum of any color other than the limited range of red reserved for ruby. Image by Henry Chaplin @ iStockphoto. |
 | Corundum (ruby) in zoisite from Merkestein, Transvaal, South Africa. Specimen is about six inches (fifteen centimeters) across. |
| Deep Blue Corundums are known as "Sapphires" © iStockphoto / mirajewel |
 | Emery rock rich in corundum and spinel from Peekskill, New York. This specimen is approximately six inches (fifteen centimeters) across. |
| Deep Red Corundums are known as "Rubies" © iStockphoto / Wilson Valentin |
| Colorful rough corundums (sapphires) from Sri Lanka. Specimens are about 1/4 inch (six millimeters) across. |
 | Common corundum showing parting and hexagonal habit from the Zoutpansberg District, Transvaal, South Africa. Specimen is approximately two inches (5 centimeters) across. |
| 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. |
|
