corindone

Corindone, also known as corundum, is a mineral species consisting of aluminum oxide (Al2O3). It crystallizes in the hexagonal system and typically forms prismatic crystals. With a Mohs hardness of 9, it ranks among the hardest natural materials and is resistant to most acids. Pure corindone is colorless; trace impurities yield a wide range of colors.

The gemstone varieties are ruby (red) and sapphire (blue), with many other colors collectively known as fancy sapphires. Color arises from trace elements: chromium produces red rubies, iron and titanium produce blue sapphires, while other impurities create pink, orange, yellow, green, or violet stones. Some colorless crystals are used as gems or abrasives after treatment.

Natural corindone forms in high-temperature, high-pressure environments in aluminous rocks. It occurs in metamorphic rocks and in igneous rocks such as nepheline syenites and pegmatites. It is found worldwide, with important sources in Sri Lanka, Myanmar, Madagascar, Tanzania, Afghanistan, and the United States.

Industrial use is dominated by its hardness: natural corindone is mined as an abrasive and occasionally as a gemstone. Synthetic corundum is also produced for industrial use and as gem-quality material. Synthetic corundum is created by methods such as flame fusion (Verneuil), flux growth, and the Czochralski process, and is used for abrasive grains, watch crystals, LED substrates, and laser components. In the gem trade, red corindone is ruby and blue corindone is sapphire; other colors are marketed as various sapphires.