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  • Sara Thompson

Same Mineral, Different Colors

By Sara Thompson

Special to the Enterprise

Image credit Wikimedia user Ra'ike


Found in all three types of rock, Corundum is formed under high temperatures and is most commonly found in metamorphic rocks. It is rated nine on the Mohs scale, which means only diamonds are harder. Its chemical formula is made up of aluminum and oxygen, Al2O3. Like any mineral, a trace amount of another element can cause the color to change slightly. Due to these traces corundum comes in a variety of colors, but two specific varieties are considered gem worthy and are the most sought after, rubies and sapphires.


Rubies are made when corundum has trace amounts of chromium. It can range from a pale red, almost pink, to a deep, blood-red color. Large ruby mines are found in several areas in Asia, Australia, and Africa, with a few smaller ones found in North America. It is possible to treat rubies with extreme heat to improve their clarity and deepen their color.


Sapphires are another variety of corundum gemstone. Unlike rubies which are classified by their red color, sapphires can come in a variety of colors, with blue being the most recognized. Trace elements of chromium, iron, magnesium, or titanium can create colors of blues, pinks, yellows, greens, purples, and even clear. Similar to rubies, sapphire mines are found in places in Asia, Africa, Australia, and even a few small North American locations. Sapphires can also undergo heat treatments to make the gemstone’s color deepen and clear.


Corundum also has an industrial use. For minerals that are not considered “gem-grade” are used for abrasive purposes. Because corundum is second only to diamonds in hardness, corundum is often used for rock saws and drilling equipment because it can scratch through other rock types. It is also used for glass cutting, grinding wheels, metal polish, and some sandpapers.


It is strange to think that some of the most popular gemstones are caused by an “impurity”, but trance amounts of other elements in minerals are common. Gemstone value is a man made construct and has caused incredible harm to people and the environment. Corundum used as both gemstones and industrial uses are beginning to be produced synthetically in labs. This reduces the ethical gray-areas of mining worldwide, reduces shipping needs, and creates a greater number of uniform stones and minerals for different uses. This also helps consumer’s pocketbooks, as synthetic minerals and gemstones tend to be less expensive than their natural counterparts.


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