In 1837, Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. Next to the diamond, the ruby is the hardest gemstone; it is also resistant to acids and other harmful substances. Because large, gem-quality rubies are so rare, the value of a fine ruby may be quadruple that of a similar-quality diamond. Natural rubies are found in a handful of sites around the world, such as Myanmar, Thailand, Sri Lanka, Afghanistan, Tanganyika, and North Carolina.
Rubies and sapphires are both composed of corundum, which is the crystalline form of aluminum oxide. They differ only in small amounts of chromium that gives rubies their characteristic red colour, with higher concentrations producing darker shades.
In addition to their decorative functions, rubies serve a broad range of practical purposes. For example, because of their hardness, they make long-lasting thread guides for textile machines. Ruby is even harder than steel, so it is an excellent bearing material for metal shafts in devices such as watches, compasses, and electric meters. Rubies have an exceptional wave-transmitting property that makes them ideal for use in lasers and masers. These are laser-like devices that operate in non-visible ranges of microwaves and radio waves.
Because many of these industrial uses demand very high-quality crystals of particular sizes and shapes, synthetic rubies are manufactured. With the exception of minor amounts of impurities, synthetic gems have the same chemical, physical, and optical properties as their natural ones. Although some are used as gemstones, about 75% of modern synthetic ruby production is used for industrial purposes.