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Occurs in Pinal County.

Chemical Formula: Al2O3

Specific Gravity: 3.95-4.10

Luster: Adamantine

Hardness: 9

Corundum is a hard and durable mineral that can occur in a variety of colors. Gem-quality corundum includes rubies (red) and sapphires (all other colors).

Next Pinal County mineral: Covellite

A Versatile Gemstone and Geological Marvel

The beauty and versatility of corundum are captivating.

It is a mineral that has left an indelible mark on human history and holds great geological significance. Corundum is renowned for its hardness, durability, and diverse colors, making it one of the most valuable and sought-after gemstones in the world.

Geological Origins of Corundum

Corundum is a crystalline form of aluminum oxide (Al2O3) and is one of the naturally occurring oxides of aluminum. Its name is derived from the Sanskrit word “kuruvinda,” which means “ruby.” Corundum forms in a variety of geological settings and is typically found in metamorphic, igneous, and sedimentary rocks.

Metamorphic rocks, such as gneiss and schist, are common hosts for corundum. These rocks undergo high-pressure and temperature conditions, causing the transformation of aluminum-rich minerals, such as aluminum silicates, into corundum crystals.

Corundum can also form in igneous rocks, particularly in aluminum-rich magmas. When these magmas cool and solidify, corundum can crystallize within them. However, most igneous corundum deposits are relatively small and not commercially viable as gemstone sources.

Sedimentary corundum deposits occur when corundum-bearing rocks are weathered and eroded, and the corundum crystals are transported and deposited in sedimentary environments.

Properties and Identification of Corundum

Corundum’s unique properties make it a highly desirable gemstone and an important industrial material. It is renowned for its remarkable hardness, ranking 9 on the Mohs scale. Only diamond, with a Mohs hardness of 10, is harder than corundum, making it an ideal material for cutting and grinding other minerals.

The mineral exhibits a hexagonal crystal system and typically forms prismatic or tabular crystals. However, corundum is also found in granular, massive, or cryptocrystalline forms. Its transparency ranges from transparent to opaque, and its luster is vitreous to adamantine, giving it a brilliant sparkle.

Corundum is known for its wide range of colors, which are due to trace elements present during its formation. Red corundum is known as ruby, while all other colors of corundum are considered sapphires. Blue sapphire is the most popular and well-known variety, but sapphires can also be found in shades of yellow, pink, green, purple, and even colorless.

Historical and Cultural Significance of Corundum

Corundum has a storied history of cultural significance and mystical associations. Its red variety, ruby, has long been associated with passion, love, and power. Ancient cultures believed that rubies bestowed protection and strength upon their wearers, making them highly valued as talismans and symbols of nobility and royalty.

In many cultures, sapphires have also been revered for their beauty and supposed mystical properties. Blue sapphires, in particular, have been associated with truth, fidelity, and divine blessings. In ancient times, sapphires were believed to have the power to protect the wearer from harm and attract divine favor.

The allure of corundum’s colors and exceptional hardness have led to its use in royal and ceremonial jewelry throughout history. Crown jewels, tiaras, and other regal ornaments often feature rubies and sapphires as symbols of power and prestige.

Industrial Applications of Corundum

Corundum’s hardness and resistance to abrasion make it invaluable for industrial applications. The mineral is used as an abrasive material in products such as sandpaper, grinding wheels, and cutting tools. Its abrasive properties are particularly useful for shaping and finishing hard materials like metals, ceramics, and glass.

Transparent varieties of corundum are also used in optical applications. Due to its excellent transparency and high refractive index, sapphire is utilized in laser windows, watch crystals, camera lenses, and various other optical components.

Corundum Mining

Corundum is mined around the world. Large-scale corundum deposits are often associated with metamorphic rocks in mountainous regions. India, Sri Lanka, and Myanmar (formerly Burma) have historically been major sources of gem-quality rubies and sapphires.

In some areas, streams beds area major source of corundum that has eroded from rocks along the waterway.

Australia is renowned for its sapphire deposits, with New South Wales being a significant producer. Madagascar, Tanzania, and Thailand are other notable sources of gem-quality sapphires.

Synthetic Corundum

Advancements in technology have led to the production of synthetic corundum, which possesses the same chemical and physical properties as natural corundum. Synthetic corundum, also known as lab-created or cultured corundum, is produced in controlled laboratory environments using various methods, including flame fusion and the Verneuil process.

Synthetic corundum is widely used in industrial applications, particularly as abrasives and in the electronics industry. In the gem market, synthetic sapphires are often used as affordable alternatives to natural sapphires.


Corundum, with its geological and cultural significance, is a gemstone of timeless allure and versatility. From the vibrant red of rubies to the captivating blue of sapphires, corundum’s colors have captivated civilizations throughout history. Its exceptional hardness and brilliance have made it a prized gemstone for jewelry and regal adornments, while its industrial applications have proven invaluable in shaping and finishing various materials.

Hardness 9 Corundom