Corundum Hardness

What Makes Corundum Hardness Second To Diamonds?

What Makes Corundum Hardness Second To Diamonds?

Corundum is a crystalline form of aluminum oxide (Al₂O₃) known for its remarkable hardness, rating 9 on the Mohs scale of common minerals, and one of the most durable minerals used industrially and as a gemstone. Typically, it takes the form of hexagonal barrel-shaped crystals or densely formed hexagonal plates.

While naturally transparent, corundum can display a variety of colors due to the presence of impurities such as iron, chromium, and titanium (responsible for the shimmering optical properties within the star sapphire).

These impurities result in one of the most sought-after stones. In their most famous gemstone forms, colored by iron and titanium, they’re known more significantly as sapphires, or when colored by chromium impurities, the resulting stones are known as rubies.

Just as impressive, beyond its appeal as a gemstone, corundum is used in various industries for its high thermal conductivity and optical transparency, making it a versatile mineral with significant economic value.

Chemical Composition

The stone’s main component, aluminum oxide (Al₂O₃) accounts for 95% of its structure. This means that each molecule of corundum is made up of two aluminum atoms and three oxygen atoms.

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In addition to aluminum and oxygen, corundum can contain small amounts of other elements as impurities, which can influence its mineral colouration. For instance, the presence of chromium gives corundum a red hue, resulting in ruby, while the presence of iron and titanium gives it a rich blue color, resulting in a sapphire. Despite these variations in color, the aluminum oxide content remains the main component of corundum.

Corundum Crystal Structure

Corundum’s structured aluminum oxide and oxygen structure make the durable crystal one of the most durable gemstones. Although pure corundum lacks a color of its own, trace elements like iron, titanium, and chromium give the gem its signature range of colors.

Exceptional Hardness of Corundum

Known for its extreme hardness, corundum ranks just below diamond, the hardest mineral on the Mohs hardness scale with an impressive rating of 9, owed to its unique crystal structure, where oxygen atoms are arranged in a hexagonal close-packing pattern, and aluminum atoms filling two-thirds of the octahedral gaps (a compound with six aluminum atoms symmetrically arranged around a central oxygen atom).

This structural arrangement not only accounts for its profound hardness but also its unusually high density for a mineral composed of relatively light elements like aluminum and oxygen.

The quality of corundum hardness makes it an ideal material for industrial applications, particularly as an abrasive material, where it is utilized in everything from sandpaper to large-scale machining tools. Its durability and resistance to scratching also contribute to its use in creating synthetic jewel watch bearings, offering a cost-effective alternative to diamonds in precision instruments. Additionally, the hardness of corundum, coupled with its rarity and beauty as a gemstone, makes it valuable in both industrial and jewelry design markets.

Friedrich Mohs Gemstone Hardness Scale

Corundum hardness holds a significant position on the Mohs Hardness Scale, ranked just below diamond, which sits at the pinnacle with a hardness of 10.

Rated at 9, corundum is twice as hard as topaz, which has a hardness of 8, and a considerably harder material than quartz, ranked at 7.

Common varieties of corundum include ruby and sapphire, known for their exceptional durability and scratch resistance. Despite being surpassed only by diamond, corundum is not simply one-tenth less hard than diamond.

Importantly, since the magnitude of difference between the Mohs Scale is not the same between each of the ratings in the sequence, the diamond is nearly four times harder than corundum, despite corundum’s ranking of 9 compared to the diamond’s ranking of 10.

Industrial Applications

The mineral composition makes corundum hardness incredibly useful in various industries because of its toughness and durability. Imagine trying to scratch a diamond; although it’s far from easy, corundum is almost as resilient.

This makes it perfect for making things like sandpaper and grinding wheels that need to withstand a high degree of wear and tear. It’s also used to line furnaces because it can handle an extraordinarily high temperature without melting.

The stone also plays a role in electronics by insulating electrical parts so they don’t short out. Plus, it’s used to help make chemicals like ammonia and methanol. So, whether it’s polishing metal, lining a hot furnace, or keeping electronics safe, corundum is a behind-the-scenes workhorse in multiple industries.

Gemstone Varieties of Corundum

Corundum is renowned for its stunning gemstone varieties, primarily rubies and sapphires, captivating the attention of jewelry lovers for centuries because of the stone’s vibrant hues and unparalleled hardness.

Rubies, with their deep red color, owe their allure to trace amounts of chromium, while the enchanting blues of sapphires result from iron and titanium impurities.

Beyond their traditional colors, corundum’s diverse composition allows for a spectrum of shades, leading to the creation of “fancy sapphires,” which can be pink, yellow, green, or purple.

These types of corundum gemstones remain highly popular for their aesthetics and durability, making them a favorite for fine jewelry. The unique properties and exquisite colors of corundum-based gemstones continue to make them a cherished part of both historical and contemporary necklaces, bracelets, and earrings.

Rubies

A corundum stone is specifically classified as a ruby when it exhibits a deep red color, which is primarily due to the presence of chromium impurities within its crystalline structure.

This distinctive red hue differentiates rubies from other varieties of gem-quality corundum, which are collectively referred to as sapphires when they display colors other than red.

The gem’s quality and vibrant coloration make it highly valued in the jewelry industry. The allure of rubies, coupled with their exceptional Mohs-scale hardness, contributes to their status as one of the most sought-after and recognizable gemstones in the world.

Sapphires

A corundum stone is classified as a sapphire primarily based on color and quality. While the mineral corundum can occur in various hues, a gemstone-quality specimen exhibiting a blue color is specifically called a “sapphire.”

The blue appearance is based on trace elements like iron and titanium within the corundum structure. In addition to the traditional blue variety, any gemstone-quality stone not red (ruby) or colorless (white sapphire) is also categorized as a “fancy sapphire,” which encompasses a spectrum of colors of corundum like pink, yellow, green, and purple.

The rarity, vibrancy, and clarity of these colored corundum stones contribute to their value and desirability in the gemstone market, making them highly revered in various types of handmade jewelry.

Industrial Uses of Corundum

Also valued in industrial applications because of their exceptional hardness, heat resistance, and chemical stability, corundum stones are used as grinding wheels and cutting tools, effectively cutting and shaping hard materials like glass and metals.

Additionally, as a resistant material in high-temperature processes, corundum is indispensable in manufacturing and construction, where durability and thermal stability are crucial.

Because of its chemical resistance, the mineral also plays a role in electronics as an electrical insulator and in the process of producing chemicals like ammonia, methanol, and hydrogen. These diverse industrial uses underline the economic significance of corundum, contributing to advancements across a number of essential industries.

Jewel Bearings in Watchmaking

Corundum’s exceptional hardness and chemical stability make it an ideal rock type for manufacturing jewel bearings in watches. These tiny, durable bearings are crucial components in watch movements, reducing friction between moving parts while enhancing the timepiece’s accuracy and longevity.

Using corundum in the form of synthetic ruby, watchmakers ensure that their timepieces can withstand the wear and tear of daily use. The choice of corundum for jewel bearings is not only driven by its physical properties but also by its smooth surface, which minimizes friction and contributes to the efficient operation of the intricate mechanisms within a watch.

Components in Ruby Lasers

The stone’s aluminum oxide (Al₂O₃), plays a pivotal role in the development of ruby lasers. The first functional laser, invented by Theodore Maiman on May 16, 1960, took a synthetic ruby crystal, a form of corundum with chromium impurities, to stimulate intense light (or Light Amplified though Simulated Emission of Radiation), causing electrons within it to jump to a higher energy level and subsequently emit photons.

These photons then interacted with other atoms in the gain medium, triggering a chain reaction that produces a concentrated, intense beam of light characteristic of lasers.

This process of incorporating chromium in the ruby allows the crystal to fluoresce (or glow) red under UV light, enhancing the intensity and quality of the laser beam. As a result, synthetic corundum, specifically in the form of ruby, is indispensable in laser applications, demonstrating its significant impact on modern technology.

Synthetic Corundum

The man-made version of the naturally occurring mineral dates back to 1837, with large-scale production beginning in 1903. Initially, synthetic stones were created to mimic rubies, but their uses have expanded considerably over time.

Today, it is employed in various industrial applications, such as making lenses for satellites, synthetic sapphire scratch-resistant glass on watches, and aircraft windows due to its high-strength optical quality.

Additionally, as a synthetic material, man-made corundum is used as a glass-grinding industrial abrasive and for metal polishing wheels.

The Verneuil process, a method of controlled growth in a chemical flame using oxygen and hydrogen, is often used to produce artificial corundum, especially for specialty products like gemstones. However, synthetic corundum is second to its natural sibling in gemology because it lacks the rarity and unique allure of naturally occurring stones.

Advantages Over Natural Corundum

Synthetic corundum offers several advantages over its natural counterpart, primarily due to its consistent quality, cost-effectiveness, and environmental benefits.

Manufactured in a controlled laboratory setting, synthetic corundum can be produced with uniformity and precision, ensuring consistent properties that are difficult to achieve with naturally occurring corundum.

This uniformity makes synthetic corundum a particularly valuable material in industrial applications, such as abrasives, watch bearings, and electronic components, where reliability and performance are critical.

As an added benefit, the production of synthetic corundum is less environmentally damaging than traditional mining processes since it bypasses the ecological disruption and pollution associated with extracting natural corundum.

Economically, synthetic corundum is more affordable and accessible since it can be produced in large quantities without relying on the depletion of natural resources. This combination of factors—consistency, affordability, and reduced environmental impact—makes synthetic corundum a highly advantageous material in various industrial sectors.

Enhancing Corundum Appearance

Enhancing the appearance of corundum often involves various treatment methods to improve its clarity and color. Heat treatment is one of the most common processes used to intensify the stone’s natural hues and enhance its transparency.

The process involves carefully controlled heating to alter the internal structure of the corundum, resulting in a more vibrant and clear stone.

Aside from heat treatment, diffusion treatment is another method to introduce additional elements into the gemstone’s crystal structure, creating new and desirable colors.

Fracture filling, another technique, involves filling fractures with a specific substance and enhancing the stone’s clarity by reducing the visibility of imperfections.

These enhancement techniques are widely acknowledged in the gemstone industry to maintain corundum as a sought-after gemstone in jewelry making while maximizing the beauty and value of each piece.

Treatments and Techniques

Another technique is diffusion treatment, where elements like beryllium are introduced into the crystal structure under heat and pressure, creating a range of colors, such as the sought-after Padparadscha sapphire. These treatments are typically performed on rough stones to avoid damaging the faceted ones with new fractures that can form during the process.

Market Impacts

The treatment of corundum stones by gemologists has significantly impacted the gemstone market by enhancing the appeal and value of these gems. Through various treatments such as heat and diffusion, gemologists improve the clarity and color of corundum stones, making them more attractive stones to buyers.

With the use of heat treatment dramatically increasing clarity and diffusion treatment introducing new colors, corundum stones find a broader market appeal, satisfying consumer demand for high-quality, colorful gemstones while maintaining a relatively accessible price point compared to untreated, naturally perfect specimens. This expanded market presence fosters increased consumer interest and supports the ongoing vitality of the gemstone industry.

Distinguishing Corundum from Imitations

Distinguishing corundum from its imitations involves examining several key characteristics that set it apart from other substances used to mimic its appearance.

Genuine corundum, whether natural or synthetic, has an absolute hardness of 9 on the Mohs scale, making it second only to diamond. This exceptional hardness is crucial in differentiating it from softer imitation materials.

Additionally, corundum’s crystalline structure is trigonal (three-dimensional in structure), and its chemical composition is primarily aluminum oxide (Al²O³), occasionally with trace elements such as chromium, titanium, or iron, which give it distinct colors, like red for rubies or blue for sapphires.

The presence of these impurities results in unique color variations that are often difficult to replicate accurately in imitations. Natural corundum also displays specific inclusions and growth patterns that are absent in synthetic versions.

Advanced gemological tools and techniques are often used to identify these subtle differences so that authentic corundum are separated from counterfeit or imitation gemstones in the jewelry market.

Market Trends

Corundum, primarily recognized as the gemstones ruby and sapphire, continues to maintain a respected position in the jewelry industry due to its unparalleled hardness and beautiful colors.

Although the market trends indicate a sustained demand for naturally sourced corundum gemstones due to their rarity and unique imperfections, synthetic and enhanced stones are in demand.

While synthetic corundum offers environmental and industrial advantages, the allure of natural corundum remains strong, particularly among collectors and connoisseurs who value its authenticity and age markings.

Additionally, the growing interest in the diverse color spectrum of sapphires, including orange, yellow, and green varieties, alongside the traditional red rubies and blue sapphires, creates an expanding palette for personalized jewelry pieces.

Summary and Conclusion

Corundum’s exceptional value as one of the hardest minerals will always be highly prized in its natural and synthetic forms. The far-reaching appeal and benefits within the jewelry industry and industrial sectors for its natural stones, as well as gem-quality synthetic corundum, underscore the mineral’s multifaceted value as one of the most popular gemstones.

Frequently Asked Questions

What is corundum, and what are its fundamental characteristics?

Corundum is a crystalline form of aluminum oxide (Al₂O₃) that is highly valued for its exceptional hardness. It ranks 9 on the Mohs scale, making it the second hardest natural mineral, surpassed only by diamond. While pure corundum is transparent, the presence of impurities such as iron, chromium, and titanium gives it a wide range of colors, resulting in its famous gemstone varieties. Beyond its use as a gemstone, corundum is also prized for its high thermal conductivity and optical transparency, making it valuable in various industrial applications.

How do rubies and sapphires relate to corundum?

Rubies and sapphires are both gemstone varieties of corundum, distinguished primarily by their color, which is influenced by specific trace elements. A corundum stone is classified as a ruby when it exhibits a deep red color, caused by the presence of chromium impurities. All other gemstone-quality corundum, including blue, yellow, pink, green, and purple varieties, are categorized as sapphires; as a result, while all rubies and sapphires are corundum, not all corundum gems are rubies, as the distinction is based on the specific color and impurities present.

Why is corundum considered exceptionally hard, and what are its industrial applications?

Corundum’s exceptional hardness, ranking 9 on the Mohs scale, is attributed to its unique crystal structure, in which oxygen atoms are arranged in a hexagonal close-packed pattern with aluminum atoms filling specific gaps. This arrangement contributes to its profound hardness and high density. Due to its toughness and durability, corundum is highly useful in various industries. It is commonly used as an abrasive material in products like sandpaper and grinding wheels, as a lining for furnaces due to its high heat resistance, as an electrical insulator in electronics, and in the production of chemicals like ammonia and methanol.

How does corundum compare to diamond in terms of hardness?

Corundum ranks nine on the Mohs hardness scale, placing it just below diamond, which holds the top spot at 10. While corundum is incredibly hard and can scratch any mineral with a Mohs hardness of 8 or less (like topaz, quartz, or orthoclase), it’s important to note that the Mohs scale is not linear. Despite being only one point below diamond, diamond is nearly four times harder than corundum in terms of absolute hardness.

What are “fancy sapphires”?

“Fancy sapphires” refer to gemstone-quality corundum that displays colors other than the traditional blue (caused by iron and titanium) or red (which are rubies). These diverse hues result from different trace impurities within the corundum structure. Fancy sapphires can come in a spectrum of colors, including pink, yellow, green, and purple, making them highly sought after for their aesthetics and durability in fine jewelry.

What is synthetic corundum, and what are its advantages?

Synthetic corundum is a man-made version of the naturally occurring mineral, with large-scale production dating back to 1903. It is created in controlled laboratory settings, often using processes like the Verneuil method. Synthetic corundum offers several advantages over its natural counterpart, including consistent quality and precision in its properties, making it highly valuable for industrial applications like abrasives, watch bearings, and electronic components. Furthermore, its production is more cost-effective and environmentally beneficial, as it bypasses the ecological disruption and pollution associated with natural mining.

How is the appearance of corundum enhanced for use in jewelry?

The appearance of corundum, especially for gemstone purposes, is often enhanced through various treatment methods to improve its clarity and color. Heat treatment is a standard process that intensifies the stone’s natural hues and transparency by altering its internal structure. Diffusion treatment introduces additional elements into the crystal structure under heat and pressure to create new colors, such as the Padparadscha sapphire. Additionally, fracture filling is used to reduce the visibility of imperfections by filling fractures with a specific substance. These techniques are widely accepted in the gemstone industry to maximize the beauty and value of corundum pieces.

Additional Reading

Wikipedia.org
Friedrich Mohs
geology.com
Naturally occurring corundum
gemdat.org
Gemological information about Corundum

References

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Corundum: Use as a Gemstone, Abrasive, Refractory – Corundum is used as a gemstone, abrasive, refractory and much more. Red corundum is a ruby, blue is a sapphire, any other color is a fancy sapphire. Synthetic corundum is used instead of natural corundum in most applications. (geology.com)

Corundum | Properties, Formation, Uses » Geology Science – 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. Co (geologyscience.com)

Corundum: Mineral information, data and localities. – The aluminum analogue of Eskolaite, Hematite, and Karelianite. The red (Cr-bearing) gem variety is called Ruby. The blue (Fe- and Ti-bearing) gem variety … (mindat.org)

What is corundum and what are its basic qualities? (eoas.ubc.ca)

Corundum – Corundum, naturally occurring aluminum oxide mineral (Al₂O₃) that is, after diamond, the hardest known natural substance. Its finer varieties are the gemstones sapphire and ruby (qq.v.), and its mixtures with iron oxides and other minerals are called (britannica.com)

Corundum – Minerals Education Coalition – Corundum is a crystalline form of aluminum oxide (Al₂O₃) with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the (mineralseducationcoalition.org)

The Corundum Conundrum Perspectives | EnvironBuzz Magazine – The conundrum around corundum. The earth is flooded with millions of variants of minerals, but despite its unique properties, the corundum group is one you may have never heard of. Did you know tw… (environbuzz.com)

CORUNDUM (Aluminum Oxide) (galleries.com)

Corundum – New World Encyclopedia (newworldencyclopedia.org)

Corundum – Details and Facts – Details and Facts for Corundum – Corundum is a very popular mineral composed of aluminum oxide. (allmineralsrock.com)