Tiffany Haney
Corundum is a crystalline form of aluminium oxide (Al2O3) that typically contains traces of iron, titanium, vanadium, and chromium. It is a naturally transparent material but can take on different colours depending on the impurities in its structure. It is one of the hardest known natural substances, with a rating of 9 on the Mohs scale, and is often used as an abrasive. Corundum is also a precious gem, with its two primary varieties being ruby and sapphire. It is widespread in nature and can be found in igneous, metamorphic, and sedimentary rocks, as well as in stream and beach sands due to its hardness and resistance to weathering. While large deposits are rare, some of the richest deposits occur in India, Myanmar, Russia, Zimbabwe, and South Africa. Corundum can be mined or synthesised, with synthetic corundum having a lower environmental impact than its natural counterpart. In terms of panning, corundum, in the form of rubies and sapphires, can be found in gold-pan concentrates along with other dense gemstones. Gold panning relies on differences in mineral density, and the gemstones that are left in the pan concentrates have specific gravities of 2.9 or higher.
| Characteristics | Values |
|---|---|
| Composition | Crystalline form of aluminium oxide (Al2O3) |
| Gem Varieties | Ruby and sapphire |
| Colour | Ruby is red, sapphire is blue |
| Hardness | 9 on the Mohs scale |
| Density | 4.02 g/cm3 (251 lb/cu ft) |
| Melting Point | 2,040° C (3,700° F) |
| Uses | Gemstone, abrasive, refractory material, mechanical parts, optics, watch crystals, instrument windows for satellites and spacecraft, laser components, ceramic armour, alloys |
| Mining Locations | Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, India |
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What You'll Learn
Corundum is a crystalline form of aluminium oxide
Corundum crystallizes in the hexagonal system, forming pyramidal or rounded barrel shapes. In the corundum crystal structure, the oxygen atoms form a slightly distorted hexagonal close packing, with two-thirds of the octahedral sites between the oxygen ions occupied by aluminium ions. The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry to R3c and the crystal class to trigonal. The structure of corundum is sometimes described as pseudohexagonal.
Corundum is a naturally occurring mineral found in igneous, metamorphic, and sedimentary rocks. It occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low-silica igneous syenite and nepheline syenite intrusives, as well as masses adjacent to ultramafic intrusives and large crystals in pegmatites. Corundum is widespread in nature, but large deposits are rare. Some of the richest deposits occur in India, Myanmar (Burma), Russia, Zimbabwe, and South Africa.
Corundum has a high density of 4.02 g/cm3 (251 lb/cu ft), which is unusual for a transparent mineral composed of low-atomic mass elements like aluminium and oxygen. It is extremely hard, with a rating of 9 on the Mohs hardness scale, allowing it to scratch almost all other minerals. This property makes corundum useful as an abrasive for grinding optical glass, polishing metals, and in sandpapers and grinding wheels. It is also used in refractories due to its high melting point of 2,040° C (3,700° F). In addition to its industrial applications, corundum is valued as a precious gem, with ruby and sapphire being its two primary gem varieties.
Synthetic corundum is produced as a specialty product for gem use through the Verneuil process, which involves slow accretion and controlled growth on a boule in an oxyhydrogen flame. Synthetic corundum has a lower environmental impact than natural corundum by avoiding destructive mining and conserving resources. It is used in various industrial applications, including the production of mechanical parts, scratch-resistant optics, watch crystals, and ceramic armour.
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It is a naturally transparent material
Corundum is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium, and chromium. It is a naturally transparent material, composed of low-atomic mass elements aluminium and oxygen. In its pure state, corundum is colourless, but the presence of small amounts of impurities can impart a broad range of hues to the mineral. For example, rubies get their red colour from chromium impurities, while sapphires exhibit a range of colours depending on the transition metal present—blue sapphires, for instance, contain titanium and iron. Corundum is one of the hardest known natural substances, with a rating of 9.0 on the Mohs scale, and is used as an abrasive. It is also mined for its use as a precious gem.
Corundum's natural transparency, coupled with its high hardness, makes it a valuable material for optics and space equipment. For instance, it is used in the production of scratch-resistant glass and other high-strength optical materials, such as lenses for satellites. The KAGRA gravitational wave detector's main mirrors are made of synthetic corundum, specifically 23 kg (50 lb) sapphires. Corundum is also used in the development of ceramic armour, thanks to its high hardness.
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low-silica igneous syenite and nepheline syenite intrusives. It is often found as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes, and as large crystals in pegmatites. Due to its hardness and resistance to weathering, it is also commonly found as a detrital mineral in stream and beach sands. The largest documented single crystal of natural corundum was measured at about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in) and weighed 152 kg (335 lb).
While natural corundum is a valuable material, its mining can be environmentally destructive and resource-intensive. Synthetic corundum, on the other hand, has a lower environmental impact by avoiding destructive mining and conserving resources. However, its production is energy-intensive and involves the use of potentially harmful chemicals. Synthetic corundum is widely used in industrial applications, such as the production of mechanical parts (tubes, rods, bearings, and other machined parts), and has largely replaced natural corundum in abrasives in the US.
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It is the second hardest known natural substance
Corundum is a crystalline form of aluminium oxide (Al2O3) that typically contains traces of iron, titanium, vanadium, and chromium. It is a naturally transparent material, but can have different colours depending on the presence of transition metal impurities in its crystalline structure. Its two primary gem varieties are ruby and sapphire. Rubies are red due to the presence of chromium, while sapphires exhibit a range of colours depending on the transition metal present.
Corundum is the second hardest known natural substance, with a hardness of 9 on the Mohs scale. This means that it can scratch almost all other minerals. Its high melting point of 2,040° C, or 3,700° F, makes it suitable for refractories. It is also used for grinding optical glass and polishing metals, and has been made into sandpapers and grinding wheels.
Corundum is widespread in nature and can be found in igneous, metamorphic, and sedimentary rocks. It commonly occurs as a detrital mineral in stream and beach sands due to its hardness and resistance to weathering. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm and weighed 152 kg.
Corundum is also used industrially, although synthetic materials such as alumina have largely replaced it in most applications. Synthetic corundum is used to produce mechanical parts, scratch-resistant optics, watch crystals, and instrument windows for satellites and spacecraft.
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It is used as an abrasive and in the development of ceramic armour
Corundum is a crystalline form of aluminium oxide (Al2O3) that occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It is also found in low-silica igneous syenite and nepheline syenite intrusives. Corundum is typically transparent but can exhibit different colours depending on the presence of transition metal impurities in its crystalline structure. Its two primary gem varieties are ruby and sapphire.
Due to its hardness, corundum is often used as an abrasive. In fact, it is the second hardest known natural substance, after diamond. Its Mohs scale rating of 9.0 means it can scratch almost all other minerals. Emery, a variety of corundum with no value as a gemstone, is a common abrasive used on sandpaper and large tools for machining metals, plastics, and wood. It is a black granular form of corundum, in which the mineral is mixed with magnetite, hematite, or hercynite. Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India.
Corundum is also used in the development of ceramic armour. Ceramic armour is used by armoured vehicles and in personal armour to resist projectile penetration. It consists of a ceramic layer on the outer surface, called the "strike face," backed up by a ductile fibre-reinforced plastic composite or metal layer. The ceramic layer serves to fracture or deform the projectile upon impact, erode and slow down the projectile as it penetrates the ceramic layer, and distribute the impact load over a larger area. Ceramics are often used where lightweight protection is important, as they weigh less than metal alloys for a given degree of resistance. The toughness of corundum is sensitive to surface roughness and crystallographic orientation.
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Corundum can be found in mineral-rich streams
Corundum is a crystalline form of aluminium oxide (Al2O3) that typically contains traces of iron, titanium, vanadium, and chromium. It is a naturally transparent material, but it can exhibit different colours depending on the impurities in its crystalline structure. For example, rubies are red due to the presence of chromium, while sapphires can be blue because of the presence of iron and titanium. Corundum is a rock-forming mineral and is often found in mineral-rich streams because of its hardness and resistance to weathering.
Corundum has a high density of 4.02 g/cm3 (251 lb/cu ft), which is unusual for a transparent mineral composed of low-atomic-mass elements like aluminium and oxygen. It commonly occurs as a detrital mineral in stream and beach sands. In addition to its presence in streams and beaches, corundum can also be found in mica schist, gneiss, and some marbles in metamorphic terranes. It is also present in low-silica igneous syenite and nepheline syenite intrusives, as well as adjacent to ultramafic intrusives and lamprophyre dikes. Large crystals of corundum can be found in pegmatites.
The largest documented single crystal of natural corundum measured approximately 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in) and weighed 152 kg (335 lb). While large deposits of corundum are rare, some of the richest deposits can be found in India, Myanmar (Burma), Russia, Zimbabwe, and South Africa. Corundum is highly valued for its hardness, making it useful for grinding optical glass and polishing metals. It is also used in sandpapers and grinding wheels, as well as refractories due to its high melting point of 2,040° C (3,700° F).
Gold panners often discard black sand, which is typically composed of iron-based minerals like magnetite, hematite, ilmenite, and chromite. However, these concentrates can contain a diverse array of minerals, including gemstones such as diamond, ruby (corundum), sapphire (corundum), topaz, garnet, spinel, and chrysoberyl. Therefore, examining pan concentrates under a loupe can reveal a range of colours and forms that aid in the identification of specific minerals and provide valuable insights into the upstream geology.
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Frequently asked questions
Corundum is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium, and chromium. It is a rock-forming mineral.
Yes, corundum can be found in mineral-rich streams. It is often found in the form of ruby and sapphire, which are its primary gem varieties.
Corundum has a variety of uses due to its hardness. It is used as an abrasive for grinding and polishing, in sandpapers and grinding wheels, and in refractories due to its high melting point. It is also valued as a precious gem.
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It is also found in igneous syenite and nepheline syenite intrusives. Some of the richest deposits of corundum are found in India, Myanmar, Russia, Zimbabwe, and South Africa.
Corundum in its pure state is colourless, but it often takes on different colours due to the presence of impurities. Ruby gets its red colour from chromium, while sapphire gets its blue shades from iron and titanium.
