79 Corundum Facts: A Precious Stone You Need To Know About!

The term corundum comes from 'kurivinda', the Sanskrit word for 'ruby'.

It is unlikely that many people would have come across the term corundum. Unless, of course, you are a geologist or an avid player of 'The Elder Scrolls V: Skyrim'.

What is corundum, and where do you find corundum? The Goldenrock Mine, located near Darkwater Crossing in Eastmarch, is the most easily accessible corundum mine in Skyrim. Although there are more veins in a number of other locations, they are unreachable at the start of the game. Blacksmiths and general goods merchants can sell corundum ingots at all levels.

But, in reality, you cannot find a Goldenrock Mine or buy corundum ingots just like that.

Even if you have not heard of corundum, you most probably would have heard of ruby and sapphire. You may have seen or heard of rubies red in color or blue, yellow, or pink sapphire stones. These are all corundum.

In fact, blue sapphire is blue corundum, and similarly are the rubies and other sapphires. Do you want to know more about this interesting stone?

Keep reading to find out the ideal geological setting where you would find sapphire and ruby corundum, what are the properties and use of these crystals, and what all different types and colors they are available in.

The Composition And Material Properties Of Corundum

Corundum is a very interesting mineral. Its composition and material properties contribute to its multiple uses.

• Corundum is a gemstone that is a crystalline form of the chemical compound Aluminum oxide mineral.
• Also known as alumina, the chemical composition of the mineral aluminum oxide is Al2O3, and it has a hexagonal crystal structure.
• Corundum has traces of titanium, iron, chromium, and vanadium. This gemstone is a rock-forming mineral.
• Primarily, corundum is formed in two different ways. It can either be formed by the metamorphosis of limestone, or it can be formed as an igneous occurrence in rocks that lack silica.
• The largest single corundum crystal ever recorded measured 25.6 in × 15.75 in × 15.75 in (65 cm × 40 cm × 40 cm) and weighed around 335.1 lb (152 kg).
• Corundum is one of the hardest materials in the world, having a Mohs hardness of nine.
• Despite being a transparent mineral, corundum is also a high-density mineral with a density of 0.14 oz (4.02 g).
• Even though pure corundum is transparent, depending on what transition metal impurity is present in its crystalline structure, it can be found in different colors.
• Corundum is durable, hard, corrosion-resistant, non-porous, and chemical resistant, with high thermal conductivity and insulating properties.
• Corundum has an adamantine to vitreous luster.
• More commonly, corundum is known by its two gem varieties, namely ruby and sapphire, rather than by its mineral name corundum. Ruby and sapphire are popular gems.
• Corundum mixtures containing iron oxide or any other mineral are known as emery.
• Despite having the same properties, chemical composition, and crystal structure, ruby and sapphire are not the same.
• Ruby and sapphire mainly differ in the transition metal impurity present in them, which gives them different colors.
• A red corundum gem variety is called ruby, whereas a blue gem variety is called sapphire.
• Red rubies get their color due to the presence of chromium impurity, and a blue sapphire gets the blue color due to the presence of both titanium and iron.
• Red rubies and blue sapphires are popular precious stones.
• Colorless corundum is called a white sapphire, and corundum with any color other than red or blue is called a fancy sapphire.
• Pink-orange-colored corundum is called a Padparadscha.
• Corundum in any color other than red, including colorless, blue, green, blue-green, white, orange, yellow, purple, yellow-green, violet, brown, pink, peach, golden amber, black, and gray, is generally, and more commonly, called sapphire.
• Green corundum is called a green sapphire, and blue corundum is called a blue sapphire.
• Sapphires are formed deep inside the Earth. They can even take millions of years to be formed.
• The purest form of sapphires is formed when the igneous rocks transform. Large crystals of sapphires are formed when the magma cools down slowly.
• Each sapphire is unique because no two sapphires that were naturally formed will look alike.
• Ruby is formed under the surface of the Earth when exposed to extreme heat and pressure.
• The red color occurs when some of the aluminum atoms are replaced by chromium. They tend to look like red rocks in the ground.
• When you breathe on sapphire, if the fog evaporates in about two seconds, it is a real sapphire. It is probably a fake sapphire if it takes more time to evaporate. This method is used to identify whether the gems are actual sapphires.
• Natural sapphires may also have tiny specks and blemishes on them.
• Blue sapphires from Myanmar, Thailand, and Australia have plagioclase, orthoclase, columbite, apatite, calcite, monazite, fergusonite, thorite, niobite, feldspars, and zircon crystals in them.
• Sapphires from Tanzania have zircon, spinel, phlogopite, biotite, pyrite, chlorapatite, magnetite, and graphite crystals in them.

Business-Related Factors

Owing to the various uses of corundum, its worth varies according to its form and purpose.

• Opaque and muddy-colored corundum is industrial grade and is used in abrasives, power tools, and sandpaper.
• Industrial grade corundum is cheap and is worth less than $1 for a carat.
• The various grades of higher quality corundum are opaque carving grade material, cabochon grade material, and facet grade material.
• The carving grade ruby has its origin in Sri Lanka (formerly Ceylon), the cabochon grade ruby has its origin in Tanzania, and the facet grade ruby has its origin in Myanmar (formerly Burma).
• The most valuable gem corundum is the facet grade ruby.
• Popular in jewelry making, a facet grade ruby can cost more than $1 million for a carat.
• Sapphires are among the most valuable natural stones. However, synthetic stones are more popular than natural stones, as they are cheaper and more affordable.
• Color and clarity are the primary factors that make a ruby desirable, whether it is faceted or uncut; the lesser the clarity, the lesser the durability, and the higher the chances of the ruby shattering on the grinding wheels.
• A larger gemstone is equally desirable and more valuable as large gems are much rarer.
• Star ruby and star sapphire gems exhibit a 'star effect' or asterism. The inclusion of rutile fibers or titanium oxide inside the corundum makes this possible.
• When the rutile inclusions are oriented to the hexagonal symmetry at an angle of 120 degrees, in cabochon corundum, the reflected light from the needles concentrates on the surface of the gemstone and form white lines intersecting at 60-degree angles. This creates a six-rayed star.
• Very rarely can you find a 12-rayed star.
• Star sapphire is more common than star ruby.

The Extraction Process

The ways in which natural corundum is formed and synthetic corundum is made are interesting. Let's see how it's done!

• Gem-quality corundum occasionally occurs in Finland, Greenland, Norway, Pakistan, the Czech Republic, Nepal, Slovakia, and Russia.
• Non-gem corundum can be found all over the world.
• Zimbabwe, Russia, Pakistan, Afghanistan, India, and Sri Lanka, are among the countries where corundum is mined for abrasives.
• It was formerly mined from dunite deposits in North Carolina in the US and from a nepheline syenite deposit in Craigmont, Ontario.
• Emery-grade corundum can be found near Peekskill, New York, and on the Greek island of Naxos.
• Synthetically, abrasive corundum is made from bauxite.
• Corundum occurs in metamorphosed crystalline limestones, dolomites, metamorphic rocks like gneiss and mica schist, and igneous rocks like granite and nepheline syenite that are low in silica.
• Gem-quality corundum such as ruby and sapphire deposits are found worldwide in placer deposits.
• The toughness, hardness, and chemical resistance of corundum allow it to survive long after other minerals have been destroyed in sediments.
• This is why alluvial sediments often contain high concentrations of corundum.
• These deposits are among the most important sources of ruby and sapphire across the world.
• Myanmar, Cambodia, India, Sri Lanka, Montana, and Afghanistan, are some of the countries that are known for their alluvial rubies and sapphires.
• In recent decades, several African areas, notably Madagascar, Tanzania, Kenya, Malawi, and Nigeria, have become key ruby and sapphire producers.
• Marc Antoine Gaudin created the first synthetic ruby in 1837 by reacting alumina with a small quantity of chromium as a colorant at a high temperature.
• JJ Ebelmen created white synthetic sapphires in 1847 by reacting alumina and boric acid.
• Frenic and Freil created crystal corundum from which it was possible to cut small stones in 1877.
• Artificial ruby was made by fusing BaF2 and Al2O3 with chromium at a temperature above 3,630 F (2,000 C). This was carried out by Frimy and Auguste Verneuil.
• By 1903, Verneuil had claimed that he could commercially make synthetic ruby using the flame fusion process.
• A two-stage alkali leaching procedure is used to extract corundum.
• The ore is first dissolved in a sodium hydroxide solution. The solution of caustic soda is then added—this aids in the breakdown of the impurities present. The final step is to wash away any leftover residue.
• Rubies are mined from stream sediments.
• Synthetic gems can be made easily using the Verneuil process by adding traces of certain color-producing ingredients to the corundum solution and allowing it to solidify into a boule with a specific shape.
• Lab-created sapphire corundum is synthetically created crystals with a high hardness second only to diamonds on the Mohs scale, making it very hard and durable.
• Because gemstones are prone to chipping, durability is an essential quality.
• The synthetic sapphire grows in cylindrical sticks known as boules and needs a time-consuming, costly, and energy-intensive manufacturing process.
• The majority of the material found in any gem corundum mine is not of gem quality.
• Significant effort is put into increasing the quality of mined corundum stones using different processes.
• Most of the sapphire and ruby crystals are heat-treated to change their colors, enhance them, and improve clarity.
• A common example would be the heat treatment of blue sapphire to enhance its deep blue color.

Uses Of Corundum

With its excellent properties and various forms, corundum is a very useful mineral.

• Corundum's hardness contributes to the abrasive properties of the mineral, and it can resist weathering.
• Both natural and synthetic, corundum is utilized in a wide range of industrial applications due to its hardness, toughness, and chemical stability.
• Industrial bearings, wafers for circuit boards, scratch-resistant windows for electronic instruments, and a variety of other products are made using them.
• Impurities are removed from crushed corundum before it is screened into evenly sized granules and powders. These could then be used for polishing compounds, sandpapers, grinding media, and other cutting applications.
• Synthetic corundum, made from calcined bauxite, has shown to be a more consistent and dependable supply of corundum. In most products, it has taken the place of natural corundum.
• The use of emery has also declined over time as manufactured abrasives like silicon carbide that are cheaper and better performing have replaced it.
• When it comes to gemstone use, ruby and sapphire are among the big four, along with diamond and emerald.
• Their demands have never declined and are, in fact, keeping on increasing so much so that it is hard to maintain a demand-supply balance between mining and their sales.
• Synthetic rubies and sapphires are much cheaper and better looking and have started to attract more buyers than natural gemstones.