Corundum

Corundum is a mineral that is primarily composed of aluminum oxide (Al2O3) and is one of the naturally occurring crystalline forms of aluminum oxide. It is a very hard mineral and is commonly used as an abrasive in various industrial applications. Corundum is also known for its high melting point and chemical inertness, making it useful in refractory applications.

corundum gneiss with sapphire
Corundum from Sackett mine, Chester, United States
Corundum from Jagdalik, Kabul Prov., Afghanistan

Corundum is found in various colors, including red, blue, green, yellow, brown, and white. The color of corundum is determined by the presence of certain impurities, such as iron, chromium, or titanium. For example, corundum that contains chromium impurities will appear red and is known as ruby, while corundum that contains iron and titanium impurities will appear blue and is known as sapphire.

Corundum is also used in the manufacturing of various products, such as semiconductors, electronic components, and optical windows, due to its high thermal conductivity and optical transparency. Additionally, corundum is used as a gemstone and is valued for its hardness and durability, with the ruby and sapphire being the most popular and well-known gemstones made from corundum.

Overall, corundum is a versatile mineral that is widely used in various industrial and commercial applications, as well as being a popular gemstone.

Name: Probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam

Mineral Group: Hematite group.

Chemical composition of Corundum

Corundum is primarily composed of aluminum oxide (Al2O3), which accounts for over 95% of its chemical composition. In addition to aluminum and oxygen, corundum may contain small amounts of other elements as impurities, which can affect its color and other properties. Some common impurities found in corundum include iron, chromium, titanium, and vanadium. The presence of these impurities can give rise to different colored varieties of corundum, such as ruby (red) and sapphire (blue). The exact chemical composition of corundum can vary depending on the source and location where it is found, but the aluminum oxide content remains the primary component in all cases.

Physical Properties of Corundum

Optical Properties of Corundum

Corundum has several important optical properties that make it highly valued as a gemstone. Here are some of the key optical properties of corundum:

1. Refractive index: The refractive index of corundum is relatively high, with a value of around 1.76. This means that light passing through the stone is bent at a relatively sharp angle, which can create a bright and sparkly appearance.
2. Birefringence: Corundum exhibits birefringence, which means that it splits light into two rays as it passes through the stone. This can create a double image effect in certain directions.
3. Pleochroism: Corundum exhibits pleochroism, which means that it can appear to be different colors when viewed from different angles. For example, blue sapphire may appear dark blue when viewed from one angle and light blue from another angle.
4. Dispersion: Corundum has relatively high dispersion, which means that it can split white light into its component colors, creating a rainbow effect.
5. Transparency: High-quality corundum is highly transparent, allowing light to pass through the stone without significant absorption or scattering.

Overall, the optical properties of corundum contribute to its beauty and value as a gemstone, particularly in high-quality specimens such as ruby and sapphire.

Formation and occurrence of Corundum

Corundum forms under high-pressure and high-temperature conditions, typically in metamorphic or igneous rocks. It can also form in placer deposits, which are sedimentary deposits that contain minerals that have been eroded from their original source rock and transported by water or wind. The formation of corundum typically involves the concentration of aluminum and oxygen-rich materials, which can react and crystallize to form corundum.

Corundum is most commonly found in metamorphic rocks, such as schist and gneiss, where it can occur as a result of regional or contact metamorphism. In these settings, corundum is typically associated with other high-temperature minerals, such as garnet and sillimanite.

Corundum can also be found in igneous rocks, such as granite and syenite, where it may occur as a result of magmatic differentiation or fractional crystallization. In these settings, corundum is typically associated with other high-temperature minerals, such as feldspar and mica.

In addition to its occurrence in rocks, corundum can also be found in placer deposits, particularly in alluvial or fluvial deposits associated with rivers and streams. In these settings, corundum is typically found as small, rounded pebbles or grains that have been eroded from their original source rock and transported downstream.

Corundum is found in many parts of the world, including Australia, Brazil, India, Madagascar, Myanmar, Sri Lanka, Tanzania, and the United States. The quality and color of corundum can vary depending on the location and the specific geological conditions under which it formed.

Types of Corundum

Corundum is a mineral that occurs in a variety of colors and forms, each with its own unique properties and characteristics. Some of the most well-known types of corundum include:

• Ruby: Ruby is the red variety of corundum and is colored by the presence of chromium impurities. Ruby is one of the most valuable and sought-after gemstones in the world, prized for its rich color and durability.
• Sapphire: Sapphire is the blue variety of corundum, and like ruby, it is also colored by the presence of impurities. Blue sapphire is the most well-known and popular form of sapphire, but sapphire can occur in a range of colors, including pink, yellow, green, and purple.
• Padparadscha: Padparadscha is a rare and highly prized variety of corundum that has a pink-orange color. The name “padparadscha” is derived from the Sanskrit word for “lotus flower,” and the gemstone is highly valued for its unique and beautiful color.
• Color-Change Sapphire: Color-change sapphire is a type of corundum that exhibits a phenomenon called “alexandrite effect,” where the color of the stone appears to change depending on the lighting conditions. These stones are typically blue in daylight and purplish-red under incandescent light.
• Star Sapphire: Star sapphire is a type of corundum that exhibits a phenomenon called asterism, where a star-shaped pattern of light appears on the surface of the stone when viewed under a single light source. This effect is caused by the presence of needle-like inclusions within the stone.
• White Corundum: White corundum is a type of corundum that is colorless or white in appearance. It is typically used as an abrasive due to its hardness and durability.
• Black Corundum: Black corundum is a type of corundum that is black or dark gray in appearance. It is typically used as an abrasive, as well as for refractory applications due to its high thermal conductivity and chemical resistance.

Gemstone uses of Corundum

Corundum is one of the most valuable and prized gemstones in the world, and its varieties are highly sought after for use in jewelry and other decorative applications. Here are some of the most common gemstone uses of corundum:

1. Ruby: Ruby is the red variety of corundum, and it is one of the most valuable and sought-after gemstones in the world. Rubies are prized for their deep red color, which is caused by the presence of chromium impurities. Ruby is typically used in high-end jewelry, such as rings, necklaces, and earrings.
2. Sapphire: Sapphire is the blue variety of corundum, and it is also highly prized as a gemstone. Blue sapphire is the most well-known and popular form of sapphire, but sapphire can occur in a range of colors, including pink, yellow, green, and purple. Sapphire is typically used in a wide range of jewelry, including rings, necklaces, bracelets, and earrings.
3. Padparadscha: Padparadscha is a rare and highly prized variety of corundum that has a pink-orange color. It is one of the most valuable and sought-after gemstones in the world, and it is typically used in high-end jewelry, such as rings and necklaces.
4. Color-Change Sapphire: Color-change sapphire is a type of corundum that exhibits a phenomenon called “alexandrite effect,” where the color of the stone appears to change depending on the lighting conditions. These stones are typically blue in daylight and purplish-red under incandescent light, and they are prized for their unique and beautiful color.
5. Star Sapphire: Star sapphire is a type of corundum that exhibits a phenomenon called asterism, where a star-shaped pattern of light appears on the surface of the stone when viewed under a single light source. This effect is caused by the presence of needle-like inclusions within the stone, and it makes star sapphire a highly valued gemstone for use in jewelry.

Overall, the exceptional hardness, durability, and beautiful color of corundum make it a highly valuable and prized gemstone for use in a wide range of decorative applications.

Industrial uses of Corundum

Corundum has a number of important industrial uses, due to its exceptional hardness, chemical stability, and heat resistance. Some of the most common industrial uses of corundum include:

1. Abrasives: Corundum is one of the hardest minerals known, making it an excellent abrasive material. It is used in a range of abrasive applications, such as sandpaper, grinding wheels, and cutting tools.
2. Refractories: Corundum is highly resistant to heat and chemical attack, making it an ideal material for use in refractory applications. It is used to manufacture refractory bricks, which are used in high-temperature industrial processes such as steelmaking and glass manufacturing.
3. Polishing and Lapping: Corundum is also used as a polishing and lapping material due to its hardness and fine grain size. It is used to polish and lap a variety of materials, including metals, glass, and ceramics.
4. Electrical Insulators: Corundum is an excellent electrical insulator, making it useful in electrical and electronic applications. It is used to manufacture insulating parts for electrical equipment, such as insulators, circuit breakers, and high-voltage bushings.
5. Catalyst Support: Corundum is also used as a catalyst support in a range of industrial processes. It is used to support catalysts in the production of chemicals such as ammonia, methanol, and hydrogen.
6. Gemstones: High-quality corundum crystals, such as ruby and sapphire, are highly valued gemstones that are used in jewelry and other decorative applications.

Overall, the hardness, heat resistance, and chemical stability of corundum make it a highly valuable material for a range of industrial applications.

Economic value of Corundum

Corundum has significant economic value due to its many industrial and gemstone uses. Here are some of the factors that contribute to the economic value of corundum:

1. Industrial uses: Corundum is widely used in a range of industrial applications, including as an abrasive, refractory material, electrical insulator, and catalyst support. These applications make corundum an important material for a range of industries, including manufacturing, construction, and electronics.
2. Gemstone value: High-quality corundum crystals, such as ruby and sapphire, are highly prized gemstones that are used in jewelry and other decorative applications. The rarity and beauty of these gemstones make them highly valuable and sought-after, with prices often reaching tens or even hundreds of thousands of dollars per carat.
3. Rarity: Although corundum is a relatively common mineral, high-quality gem-quality corundum is relatively rare. This rarity contributes to the high value of corundum gemstones.
4. Mining and processing: The mining and processing of corundum require significant investment in equipment, labor, and infrastructure, which can also contribute to the economic value of corundum.

Overall, the combination of industrial and gemstone uses, rarity, and mining and processing costs contribute to the economic value of corundum. The global market for corundum is significant, with estimates suggesting a market size of several billion dollars annually.

Interesting facts about Corundum

Here are some interesting facts about corundum:

1. Corundum is one of the hardest minerals in the world, with a Mohs hardness rating of 9. This makes it an ideal material for use as an abrasive.
2. The red variety of corundum is called ruby, while the blue variety is called sapphire. Other colors of corundum, such as yellow, pink, and green, are referred to as fancy sapphires.
3. The name “corundum” is believed to come from the Tamil word “kurundam,” which means “ruby.”
4. The largest single corundum crystal ever found was discovered in Zimbabwe in 1958. It weighed 3,546 carats and measured 205mm x 163mm x 100mm.
5. Corundum is found in many different types of rocks, including granites, basalts, and metamorphic rocks.
6. In ancient times, rubies were believed to have protective powers and were often worn by warriors going into battle.
7. The color of corundum can be altered through the process of heat treatment, which can improve the clarity and intensity of the stone’s color.
8. Corundum has high thermal conductivity, making it useful as a material for thermal insulation.
9. Star sapphires exhibit a phenomenon known as asterism, where a six-pointed star appears on the surface of the stone when viewed under a single light source.
10. Corundum is used in the manufacturing of LED lights, as the material has high transparency and durability.

Distribution

Corundum is a widely distributed mineral found in many parts of the world. It is found in many different types of rocks, including granites, basalts, and metamorphic rocks. Here are some of the major locations where corundum is found:

1. India: India is one of the largest producers of corundum in the world, with deposits found in the states of Tamil Nadu, Andhra Pradesh, and Karnataka.
2. Madagascar: Madagascar is known for producing high-quality corundum, particularly sapphires. The deposits are found in several areas throughout the country, including Ilakaka, Ambondromifehy, and Sakaraha.
3. Sri Lanka: Sri Lanka is another major producer of corundum, particularly blue sapphires. The deposits are found in several areas throughout the country, including the Ratnapura district.
4. Australia: Australia is a significant producer of corundum, particularly in the form of sapphires. Deposits are found in several areas, including New South Wales, Queensland, and Victoria.
5. Myanmar (Burma): Myanmar is known for producing some of the world’s finest rubies and sapphires. The deposits are found in several areas throughout the country, including Mogok, Mong Hsu, and Kachin State.
6. United States: Corundum deposits are found in several locations throughout the United States, including North Carolina, Montana, and Wyoming.
7. Thailand: Thailand is known for producing high-quality rubies and sapphires, particularly in the Chanthaburi province.
8. Tanzania: Tanzania is a relatively new source of corundum, with deposits found in the Umba Valley and the Tunduru district.

Overall, corundum is found in many different parts of the world, and its widespread distribution contributes to its economic value and importance in many industries.

FAQ

Q: What is corundum?

A: Corundum is a mineral that consists of aluminum oxide (Al2O3). It is a very hard mineral, with a Mohs hardness rating of 9, and is commonly used as an abrasive.

Q: What are the different colors of corundum?

A: The most well-known colors of corundum are ruby (red) and sapphire (blue). However, corundum can also occur in other colors, such as yellow, pink, green, and purple.

Q: How is corundum formed?

A: Corundum is formed through the process of metamorphism, which involves the high-pressure and high-temperature alteration of pre-existing rocks. It can also form in igneous rocks through the process of fractional crystallization.

Q: Where is corundum found?

A: Corundum is found in many parts of the world, including India, Madagascar, Sri Lanka, Australia, Myanmar, the United States, Thailand, and Tanzania.

Q: What is corundum used for?

A: Corundum is used for a variety of industrial and commercial purposes, including as an abrasive, in the manufacture of refractory materials, in the production of LED lights, and as a gemstone in jewelry.

Q: How is corundum different from diamond?

A: Corundum and diamond are both hard minerals, but diamond is harder than corundum and has a Mohs hardness rating of 10. Diamond is also a different mineral altogether, consisting of pure carbon, while corundum is composed of aluminum and oxygen.

Q: What is the most valuable color of corundum?

A: The most valuable colors of corundum are typically red (ruby) and blue (sapphire), with high-quality stones commanding high prices in the gemstone market.

Q: How can I tell if a stone is corundum?

A: Corundum is relatively easy to identify due to its hardness and high refractive index. A simple scratch test using another mineral with a lower hardness rating can be used to determine if a stone is corundum. Additionally, corundum typically has a characteristic crystal shape and can exhibit pleochroism and birefringence when viewed under a microscope.

Q: Can corundum be treated or enhanced?

A: Yes, corundum can be treated or enhanced to improve its appearance or color. One common treatment is heat treatment, which can improve the clarity and color of certain types of corundum. Other treatments include diffusion treatment, which introduces other elements into the crystal structure to create different colors, and fracture filling, which involves filling surface-reaching fractures with a substance to improve the stone’s clarity.

Q: Is corundum a rare mineral?

A: Corundum is not a rare mineral, as it is found in many parts of the world and is relatively abundant. However, high-quality gemstone specimens of corundum can be rare and valuable, particularly in certain colors or with exceptional clarity.

Q: Can corundum be synthesized in a laboratory?

A: Yes, corundum can be synthesized in a laboratory using a variety of methods. Synthetic corundum can be used for a variety of industrial and commercial purposes, including as an abrasive and in the production of LED lights. However, synthetic corundum is not typically used as a gemstone due to its lack of rarity and uniqueness.

Q: What is the largest corundum ever found?

A: The largest corundum crystal ever found weighed approximately 750 pounds (340 kilograms) and was discovered in the Ratnapura district of Sri Lanka in 1902. The crystal was a blue sapphire and was named the “Star of Adam.”

Q: What is the history of corundum in jewelry?

A: Corundum has been used as a gemstone in jewelry for centuries, with ruby and sapphire being highly valued by royalty and the wealthy throughout history. The ancient Greeks and Romans prized corundum for its beauty and believed that it had protective powers. In the Middle Ages, ruby and sapphire were used in religious jewelry, and during the Renaissance, they were popular among the nobility.

Q: What is the difference between synthetic and natural corundum?

A: Synthetic corundum is produced in a laboratory, while natural corundum is formed through natural geological processes. Synthetic corundum can be used for a variety of industrial purposes, while natural corundum is typically used as a gemstone in jewelry. While synthetic corundum can have a similar chemical composition and physical properties to natural corundum, it is not considered as valuable or unique as natural specimens due to its lack of rarity and distinctiveness.

Q: Can corundum be found in meteorites?

A: Yes, corundum has been found in meteorites, particularly in those originating from the moon and Mars. These corundum crystals are typically much smaller than those found on Earth and are often included within other minerals.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019). Corundum: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/min-727.html [Accessed 4 Mar. 2019].