Mineral of the Week: Corundum

By Kristina Kassis (Fall 2012 GE 101 student)

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Most people, when asked to name minerals, can easily recognize commonly known minerals such as Gold, Silver, Sulfur, Carbon, Iron, Zinc, Tin, Calcium, Potassium, Hydrogen and others.  However, how many of you reading this have heard of a mineral known as Corundum?  My guess would be very few. Corundum is an exceptionally hard and durable form of aluminum oxide (Al2O3).   With a hardness of 9 on Mohs Hardness Scale, Corundum is in fact the third hardest mineral, with only Diamond and Moissinite surpassing it in hardness. Corundum can be found in igneous rocks that are very high in Aluminum but are very low in Silicon, such as Pegmatite.  Corundum can also be found in metamorphic rocks that were formed through contact metamorphism. For example, marble, hornfels, Schist, and gneiss (both forms of metamorphosed shale) are often found to contain very small traces of corundum. Because Corundum is so durable and resistant to corrosive acids and chemicals, it is able to persist long after many other elements have been eroded, so it is commonly found in beach sands and in other similarly harsh environments.

In addition to its extraordinary hardness and durability, Corundum has many other distinguishing physical characteristics. In terms of color, Corundum is frequently gray, but can also be found in white, brown, red, blue, yellow, green, purple and a variety of other colors. When scratched across a streak plate, Corundum produces a distinct white streak. It has a shiny to almost glassy luster, but is not metallic. Corundum is not opaque, but rather typically transparent to translucent. The crystals in Corundum are fine and hexagonal in shape. These crystals are usually elongated and striated crosswise and often form in thin plates.  Corundum has no natural cleavage planes, but does have conchoidal fracture, mineral fracture in which the indentation is rounded and resembles the shell of a bivalve.  All together, these physical characteristics of Corundum in conjunction with chemical properties such as whether or not it reacts with acid (Corundum does not) make it a unique mineral and therefore help geologists to determine its identity in nature.

Clearly, Corundum is unique for its exceptional hardness and durability. One of the most notable facts about Corundum however, is that it can also form not one, but TWO types of precious gemstones: rubies and sapphires, the birthstones for the months of July and September, respectively.  In fact, the name “Corundum” is derived from the Tamil word kuruntam meaning “ruby”, and related to Sanskrit kuruvinda.  Rubies are formed when small traces of Chromium contaminate Corundum and stain it a deep red. Sapphires can be found in many colors, but are most commonly light to dark blue and are also formed by contamination of other minerals, specifically Titanium and Iron in Corundum. Corundum that contains very small amounts of Titanium is completely colorless. However if a similar amount of Iron is present instead in the corundum, it may appear pale yellow in color. In addition, if both Titanium and Iron impurities are present together within the Corundum, the result is a striking deep blue Sapphire. These sapphires are the most valuable.  Notable deposits of rubies and sapphires occur in Burma, Cambodia, Sri Lanka, India, Afghanistan, Myanmar, Thailand, Vietnam, Pakistan, China, Australia, Kenya, Tanzania, Nigeria, and Malawi.  In fact, much of Africa has recently become a significant producer of Corundum, especially in Madagascar. Even the United States has deposits of Corundum. For example, Montana has a large quantity of Ruby and Sapphire deposits. So despite its obscure name, Corundum can be found all over the world!

Due to its exceptional hardness, Corundum is most commonly used as an abrasive. It is crushed to a powder of varying size depending on how rough the grinding stone, cutting tools or sanding paper needs to be.  Emery, the most common form of natural corundum used to manufacture abrasives, is a granular metamorphic or igneous rock that is very rich in Corundum.  Emery is a conglomerate of many different oxide minerals, typically Corundum in conjunction with magnetite and hematite.  Apart from ornamental uses and abrasives, synthetic corundum is also used to produce mechanical parts as well as scratch-resistant glass and laser and spacecraft components because it is resistant to UV light. Clearly, Corundum is not only a durable, but also extremely versatile element.

Surprising right? It turns out that most people are actually familiar with Corundum, but few know it by its mineral name. So, July and September babies, next time someone asks you what your birthstone is, present them with a conundrum and tell them its corundum!


[1] “The Mineral Corundum.” The Mineral and Gemstone Kingdom. Hershel Friedman and Minerals, n.d. Web. 30 Oct 2012. <http://www.minerals.net/mineral/corundum.asp&xgt;.

[2] “Corundum Mineral Data.” Mineral News: The Mineral Collector’s Newsletter. N.p Web. 30 Oct 2012. <http://www.webmineral.com/data/Corundum.shtml

[3] “Blue Sapphire.” Causes of Color. N.p Web. 30 Oct 2012. <http://www.webexhibits.org/causesofcolor/8.html&gt;.

[4] “Corundum.” Geology.com: News and Information about Geology. Geology.com, n.d. Web. 30 Oct 2012. <http://geology.com/minerals/corundum.shtml&gt;.

[5] “Corundum Uses.” Gemstone Advisor: Your Advisor on Gemstones and Gemstone Jewelry. N.p n.d. Web. 30 Oct 2012. <http://www.gemstonesadvisor.com/corundum-uses/&gt;.

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