Corundum is a rock-forming mineral. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha (sapphire) if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen. The name "corundum" is derived from the Tamil word KURUVINDAM or Sanskrit word KURUVINDA meaning ruby.
Because of corundum's hardness, it can scratch almost every other mineral. It is commonly used as an abrasive on everything. Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka and India. Historically it was mined from deposits in North Carolina, USA and in Craigmont, Ontario, Canada. Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China. The surfaces of the axes are remarkably smoothly polished.
In 1837, Marc Antoine Gaudin made the first synthetic rubies. The Verneuil process developed in 1902 by the French chemist Auguste Verneuil, also called flame fusion, allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger size than normally found in nature.
Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instruments, windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components.
