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In the mid-1800s, watch makers in Switzerland needed tiny bearings that were highly resistant to abrasion. They discovered that they could drill a hole into a tiny piece of corundum and use it for a smooth-running, long-life bearing. The corundum was much harder than the metals used to make the moving parts of a watch,…

In the gemstone and jewelry market, almost all of the attention goes to a small group of gems known as “the big four”: diamond, ruby, sapphire, and emerald. Two of these, ruby and sapphire, are gem corundums. These most popular gems are highly sought after and have been mined in many parts of the world for thousands of years. Today, millions…

Emery stone is a granular metamorphic or igneous rock that is rich in corundum. It is a mixture of oxide minerals, typically corundum, magnetite, spinel and/or hematite. It is the most common form of natural corundum that has been used to manufacture abrasives. The use of emery as an abrasive has declined significantly in the last several decades. It…

The extreme hardness of corundum makes it especially useful as an abrasive. Crushed corundum is processed to remove impurities and then screened to produce uniformly sized granules and powders. These are used for grinding media, polishing compounds, sand papers, grinding wheels, and other cutting applications. Some problems with using natural corundum as an abrasive are…

Corundum is found as a primary mineral in igneous rocks such as syenite, nepheline syenite, and pegmatite. Some of the world’s most important ruby and sapphire deposits are found where the gems have weathered from basalt flows and are now found in the downslope soils and sediments. Corundum is also found in metamorphic rocks in locations where aluminous shales or bauxites have…

Corundum is an exceptionally hard and tough material. It is the third-hardest mineral, after diamond and moissanite. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale. Its hardness, high specific gravity, six-sided crystals and parting are very good diagnostic properties to use in its identification. A summary of the physical properties…

Most people are familiar with corundum; however, very few people know it by its mineral name – instead they know it by the names “ruby” and “sapphire.” A gemstone-quality specimen of corundum with a deep red color is known as a “ruby.” A gemstone-quality corundum with a blue color is called a “sapphire.” Colorless corundum is known…

Corundum has historically been used as an abrasive, but it is most famous as the mineral of ruby and sapphire. What is Corundum? Corundum is a rock-forming mineral that is found in igneous, metamorphic, and sedimentary rocks. It is an aluminum oxide with a chemical composition of Al2O3 and a member of the trigonal crystal system. The mineral is widely known for its extreme hardness and for…