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A red, transparent variety of the mineral corundum, the ruby is a highly prized precious stone which has been sought after for many centuries. It is second only to diamond in hardness and large clear rubies from Burma and Thailand are much more valuable than diamonds of equivalent size.
The color of a ruby varies from specimen to specimen. Some are rose red, others 'ruby' red, others carmine and some a deep purplish red called 'pigeon's blood red'.
Clear, dark specimens are the most highly prized. Some specimens are cut in a convex form without facets (cabochon) and they display what is called asterism. This refers to the presence of a six-rayed star visible in the interior of the stone. Rubies with this property are termed 'star rubies' and are also very highly prized.
The finest specimens of ruby come from Upper Burma near the town of Mogok, where they have been mined since the fifteenth century. Other important deposits are found in Thailand, Afghanistan, Sri Lanka, China and in the Ural Mountains in Russia.
In 1837 synthetic rubies were produced for the first time by fusing alum and chromium oxide pigment at high temperatures. Improvements in their manufacture made since the beginning of the twentieth century have made possible the production of synthetic rubies. These are so like natural ones in their physical and chemical properties that an experienced eye is needed to distinguish between them. Air bubbles, which are formed and trapped during the cooling of a molten mass of synthetic ruby, tend to be perfectly spherical in shape. In natural crystals bubbles are usually irregular in shape. Striations present in the natural stones take the form of straight lines whereas those in synthetic rubies are generally curved.
In addition to their importance as highly prized gems, rubies are used in industry as jewels in watches and as bearings in many scientific instruments. Synthetic rubies are also used as gem stones but in contrast to the natural stone about 75 per cent of the annual production of synthetic rubies is used in watch and instrument making.