Corundum is a mineral found in many metamorphic, sedimentary and igneous rocks. It is aluminum- based rock-forming mineral. Pure corundum ore exists in colorless form. The mineral gets its color majorly from iron and chromium impurities. Some other mineral impurities such as cobalt, nickel, and vanadium can also be found in the deposits of corundum. The mineral is known to possess high levels of hardness, therefore, it can easily be found in crystalline form in many different colors.
Even though corundum has been used by humans in different shapes and forms for centuries, the revolution in modern mining history of corundum happened when a geologist W.F. Ferrier pointed to the large deposits of corundum in Canada in his survey report in 1896. In a few years, the mining of corundum started in the communities of Ontario, Hastings colony and Carlow. Now corundum, both in natural and synthetic form is used for many purposes.
People know corundum by sapphires and rubies
People are familiar with this mineral through the gemstones of ruby and sapphires. Both of these precious stones are the variants of corundum. Finer grades of corundum in red hues are called rubies while the ones with blue shades are known as sapphires.
Physical properties of corundum
Corundum comes in the top three hardest minerals along with moissanite and diamond. It is assigned with that hardness of 9 on Mohs scale. They get recognized by their distinctive physical properties. They possess high specific gravity and definite hexagonal crystalline structure. They also have a great tendency to cleave along stressed areas.
The history associated with the name ‘Corundum’
Most historians agree to the fact that the word ‘Corundum’ is derived from the Sanskrit word korund or Kurundum. Another fable associates the name with an ancient district of Karund in the state of Orissa, India. The deity of the region is Manikeswari, which literally means ‘goddess of rubies or corundum’. This claim was also substantiated by the fact that the karund district was known for the deposits of the ruby gemstone.
The occurrence of corundum in Nature
Corundum can be found in many different geological settings and in all those rocks that are rich in aluminous instead of silica. Syenite, nepheline syenite and pegmatite, all of these igneous rocks contain corundum as their primary mineral. It is also found in metamorphic rocks derived from aluminous and carbonate sediments. Marble, schist and other products of regional metamorphism also contain corundum.
Some of the naturally occurring marble deposits contain gem-quality of vibrant colors and clarity. Due to its relatively inert nature and hardness, it is able to survive the extremes of weather and can be found in different alluvial deposits. Most of the higher grade corundum is obtained from the clay beds of different river deltas. Word’s famous alluvial deposits of corundum are located in Cambodia, Sri Lanka, Afghanistan and India.
Corundum hardness: best suited for abrasives
The hardness of corundum makes it the best contender for abrasives for many industrial uses. Corundum is first crushed into smaller particles which are then processed to produce same-sized granules. The final product is used in different ways for the purpose of abrasion. It is used to make polishes, sandpapers, grinding wheels and other grinding media.
However, with more innovation and precision in the manufacturing process, some disadvantages of natural corundum started to appear. The particles of corundum are usually small and irregular in their sizes – this means the quality needed in the making of many products is not necessarily met.
Synthetic corundum, on the other hand, has consistent physical properties. It is made of calcined bauxite. Corundum in the form of emery rock has also been used as an abrasive in many industries. It is a granular rock rich in corundum, spinel, magnetite and hematite.
Corundum as a gemstone
Even though the use of corundum as an abrasive has declined in recent years due to the availability of better options, no one can take the place of corundum in the world of gem and jewelry because two of the ‘big four’ minerals are variants of corundum.
Diamond, ruby, sapphire and emerald due to their historical significance and aesthetic appeal are the four most sought-after gemstones in the world.