Corundum: The Strangely Prolific Mineral

January 25, 2018
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Faberge sapphire necklace

Did you know that corundum is exactly half of everything you could ever want in a gemstone? The “big four” gemstones in the jewelry industry are diamond, ruby, sapphire and emerald. Both ruby and sapphire are varieties of corundum, so although you may have never heard of it, the mineral we are discussing today is a surprisingly big part of your life. And yes, I understand that rubies and minerals probably don’t play a big role in the lives of most readers of this blog, but how about watches, sand paper, nail files, lasers and computers? The fact is that if corundum has played a vital role in many industries that effect our lives everyday, so let’s talk about some of the ways that a mineral you’ve never heard of has affected you.

First off, let’s cover corundum’s most glamorous job: gemstone. Both sapphire and ruby are varieties of corundum, they both have the chemical formula Al2 O3. Rubies get their red color from small amounts of chromium added to the chemical formula and sapphires get their blue color from small amounts of iron and titanium. Corundum comes in a wide variety of colors depending on the individual mineral’s chemical makeup, specimens colored anything but red or blue are referred to as “fancy sapphires”. Sapphires are one of the few things on earth that I am most likely to own a “fancy” version of, because fancy sapphires are often cheaper than rubies and  regular sapphires.

Ruby specimen from our Cullen Hall of Gems and Minerals.

Aside from its beauty and rarity, corundum is also valued for its hardness. The mineral has a hardness of 9 on the moh’s hardness scale, just under diamond. Since corundum has historically been cheaper than diamond, it was chosen as the material to create “jewel” watch bearings. The reason a hard mineral was needed to create watch bearings is because friction is a big problem in mechanical watches. Not only does friction hurt the efficiency of the movement, but also direct contact between metal gears of the same hardness will eventually wear them down. Originally natural gemstones were used used in the movements, but in the early 1900’s a new synthetic production process was developed and now only specially produced synthetic corundum is used. 

It’s hardness is also why corundum used to be used in nail files and sand paper (nowadays other materials have replaced corundum in these products) and even today the mineral is used to make “crystal” watch faces. Also, if you ever wondered why the glass windows in grocery store scanners don’t shatter when you drop cans on them, it’s because they’re made out of corundum. The mineral’s resistance to heat also makes it useful in the construction of kilns.

An interesting quality of high-grade rubies is that the chromium incorporated into their chemical makeup makes them fluoresce under UV light. This means that under normal lighting conditions, UV energy is absorbed and re-emitted at a lower energy level, in the red region of the spectrum, increasing the intensity of the gem’s red color. This quality of ruby was exploited when the first laser was invented by Theodore Maiman in 1960. 

There really are few aspects of our lives that have not somehow been affected by corundum. It’s the gemstone that everybody desires but no one has heard of; it set the pace of our days right up until the start or the digital age and grocery stores would be a much deadlier place without it. 

If you want to learn more about corundum and all the other amazing minerals that make up our world, come visit our Cullen Hall of Gems and Minerals at HMNS!

Authored By Chris Wells

Adventure is my middle name. Well… actually it’s French. Literally, it’s Christopher French Wells. But the spirit of adventure lives in me, and has always inspired me to go out and seek new experiences. I’ve traveled to Europe, Mexico and South America, as well as few places in the U.S. I’ve seen different places with different cultures, learned some things about humanity and about myself in particular. My goal is to lend my unique perspective, carved out of my own triumphs and tragedies, fears and fancies encountered during my years of college and international travel, to the other great voices of this blog. Hopefully to the enjoyment of our readers…

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