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What is Corundum?

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Corundum is a naturally occurring aluminium oxide mineral that is the second hardest natural substance after diamond. The finer types are sapphire and ruby, and its mixtures of iron oxides and other minerals are referred to as emery. Corundum mineral is colourless in its pure form, but the presence of trace amounts of impurities will add a wide range of hues to the mineral. Ruby gets its red colour from chromium, while sapphire gets its blue shades from iron and titanium; most corundum contains approximately 1% iron oxide. Other aluminous minerals that weather easily include margarite, zoisite, sillimanite, and kyanite. 


Properties of Corundum

Corundum is a mineral of  aluminum oxide that is typically white, grey, or brown in appearance, but gem colours include red ruby, blue, green, yellow, orange, violet, and pink sapphire. Colorless forms can also be used. Let's go over Corundum's physical properties in depth:

  • Gray to brown is the most common colour. When pure, it is colourless, but trace amounts of different metals create almost any colour. Chromium produces red (ruby), and iron and titanium combinations produce blue (sapphire).

  • It has no colour (harder than the streak plate).

  • Luster ranges from adamantine to vitreous.

  • The specific gravity ranges from 3.9 to 4.1 (very high for a nonmetallic mineral).

  • It has a high specific gravity, hardness, hexagonal crystals that often taper to a pyramid, parting, lustre, and conchoidal fracture.

Corundum Uses

Corundum has been largely replaced in most industrial applications by synthetic materials such as alumina, an aluminium oxide derived from bauxite. Slow accretion and regulated growth on a boule in an oxyhydrogen flame will produce artificial corundum as a specialty product for gem usage. The Verneuil method is the name given to this operation. Let’s look at some more Corundum uses: 

  • As a gemstone it is used. 

  • Because of its hardness, it is used as an abrasive.

  • It's used for polishing and sanding of optical lenses.

  • Because of its high melting point (2,040 ° C or 3,700 ° F), it is often used in refractories.

  • It's used to grind optical glass and polish metals, and it's even been turned into sandpaper and grinding wheels. It has also been used in refractories due to its high melting point.

  • Watches also make use of colourless synthetic sapphire. Its longevity, vitreous lustre, and scratch resistance make it an ideal transparent covering for the face of a mechanical or digital device.

Corundum has a wide range of applications. It is chemically inert and heat resistant. Because of these properties, it is ideal for making refractory items such as fire stone, kiln liners, and kiln furniture. These items are now typically made of synthetic corundum. Pure corundum is colourless, translucent, long-lasting, and resistant to scratches. Large crystals of clear synthetic corundum are grown, sawn into thin sheets, and then used as the windows of grocery store scanners, watch crystals, aircraft windows, and electronic device protective covers.


Corundum Facts

  • Almost all of the attention in the gemstone and jewellery industry is focused on a select group of gems known as "the big four": diamond, ruby, sapphire, and emerald. Ruby and sapphire are also gem corundums.

  • Corundum crystallises in the hexagonal structure and takes the form of pyramidal or rounded barrels. It is common in nature, appearing in igneous, metamorphic, and sedimentary rocks.

  • Watchmakers in Switzerland required tiny abrasion-resistant bearings in the mid-1800s. They discovered that by drilling a hole in a tiny piece of corundum, they could create a smooth-running, long-life bearing.

  • The corundum was much stronger than the metals used to produce a watch's moving parts, and it could withstand constant abrasion without failing. After their gemstone counterparts, corundum bearings were dubbed "jewel bearings."

  • Corundum is used as a mineral in metamorphic terranes as mica schist, gneiss, and some marbles. It's also found in igneous syenite with low silica and nepheline syenite intrusives. Other locations include masses near ultramafic intrusives, lamprophyre dikes, and massive crystals in pegmatites. Because of its hardness and weathering resistance, it is commonly used as a detrital mineral in stream and beach sands.

How is Corundum Valued?

Rubies and sapphires are valued first and foremost for their colour, followed by clarity. As all rubies and sapphires are valued for their vivid colour, it is unsurprising that this is the primary determining factor for their value. A clean stone is more pleasing than one with a lot of inclusions. The origin of a stone has a significant influence on its value; size and cut are also important considerations.


Stones found in conflict zones or unknown areas are often unappealing to many customers. Corundum sourced from historical sites, on the other hand, is comparable to brand name products such as Gucci or Armani. When it comes to scale, larger stones are more rare. In terms of cut, a badly faceted stone will not display its best colours and will most likely need to be re-cut, resulting in a loss of carat weight. As with diamonds, there are 4+1 Cs to evaluating gem corundum: colour, transparency, cut, carat, and country of origin!


The purity and strength of the blue in the best sapphires are valued, with "ideals" displaying either a refreshing cornflower blue or a velvety royal blue. The majority of high-quality sapphires come from three different parts of the world. Kashmiri stones are also the most expensive and have an intense, velvet-like blue. Burmese sapphires are also valued for their deep blue colour. Burmese stones are known for their beautiful asterisms. Finally, Sri Lanka and its cornflower blues, not to mention their enormous scale, are highly sought after.


Difference Between Corundum and Ruby

The corundum gemstone family includes ruby and sapphire. Corundum is very compact and dense, with no gemstone cleavage. It is also the second hardest natural mineral after diamond in terms of hardness. Corundum of gem quality is also very unusual. Because of these characteristics, both types of corundum are among the most sought-after jewellery stones. The red variety of corundum is known as ruby. Sapphires are all other shades of corundum, including colourless corundum.


Conclusion

Since corundum is a metamorphic variant of bauxite, it is most commonly found as a metamorphosed bauxite deposit or as altered aluminous shale. It has an exceptionally high density and is almost as hard as diamond. Corundum of gem quality is found in streams because of its high density, which allows pieces to deposit within one local area, and its hardness, which makes it resistant to weathering. Corundum is derived from the Sanskrit word, kuruvinda which means "ruby," and is the name given to red corundum. Corundum gems include ruby and sapphire. Corundum is an aluminium oxide that is usually white, grey, or brown in appearance, but gem shades include red ruby, blue, green, yellow, orange, violet, and pink sapphire. Colorless forms can also be used. Ruby and pink sapphire form a continuous colour succession; only stones with darker hues are called rubies. Corundum crystals are usually hexagonal, tabular, tapering barrel-shaped, or dipyramidal in form. Corundum may be both massive and granular. It can be found in syenites, pegmatites, and high-grade metamorphic rocks. It is found in placer deposits.

FAQ (Frequently Asked Questions)

1. What is Corundum?

Ans: Corundum is a crystalline type of aluminium oxide with traces of iron, titanium, vanadium, and chromium. It is a mineral that contributes to the formation of rocks. It is also a naturally transparent material, but the presence of transition metal impurities in its crystalline structure causes it to change colour. Corundum is the second hardest natural substance after diamond. The finer types are sapphire and ruby, and its mixtures of iron oxides and other minerals are referred to as emery.

2. How is Corundum Recognized and Distinguished from Other Materials?

Ans: Corundum's hardness of 9 identifies it from other minerals, as does its hexagonal structure. However, both of these characteristics are more useful in the field than in the jewellery shop, where it is perfectly acceptable to scratch the stone. Since corundum always occurs in certain rock types with certain other minerals, the geological setting will often rule out or reinforce the likelihood of sapphire or ruby. Corundum's mineral and fluid inclusions, low dispersion, high density, and two refractive indices are critical for identification when cut (it is dichroic). 

3. What is Corundum Used for?

Ans: Because of its extreme hardness, corundum is used as an abrasive in addition to being a precious gem. Corundum has many other applications. It is chemically inert and heat resistant. Because of these properties, it is ideal for making refractory items such as fire stone, kiln liners, and kiln furniture. These items are now typically made of synthetic corundum. Pure corundum is colourless, translucent, long-lasting, and resistant to scratches. Large crystals of clear synthetic corundum are grown, sawn into thin sheets, and then used as the windows of grocery store scanners, watch crystals, aircraft windows, and electronic device protective covers.