Ruby has been the world's most valued gemstone for thousands of years. In the ancient Sanskrit language, it was called ratnaraj or "king of precious stones". Even today, rubies are considered more valuable and rare than even the top quality colorless diamonds.

These gemstones are red variations of the mineral corundum, a crystalline form of aluminum oxide and one of the most durable minerals that exists; only diamonds are harder. The rich red color arises from the substitution of a small number of aluminum atoms by chromium atoms. Pure corundum is a colorless, trigonal crystal, but it occurs in a wide variety of colors due to infiltrations of other elements. All color variations of corundum, with the exception of ruby, are called sapphires.

Rubies vary widely in color, ranging from rose red through so-called ruby red and carmine to a deep purplish red, called pigeon blood -- the most highly prized color variation. When cut into a cabochon (a convex, unfaceted form), some specimens of ruby exhibit asterism; that is, a six-rayed star can be seen in the interior of the stone. Such stones are called star rubies. When exposed to high temperatures, rubies turn green, but they regain their original color after cooling. Some rubies phosphoresce with a vivid red glow when illuminated by ultraviolet light.

The most famous, and possibly oldest, source of fine rubies is Burma, now called Myanmar. Stone Age and Bronze Age mining tools have been found in the ancient mines of Mogok. Rubies are currently being mined in other areas of Southeast Asia, as well as Africa and other parts of Asia.

Many gemstones that are not rubies are also called rubies. The balas, or balas ruby, for example, is a type of spinel. The Bohemian ruby is derived from rose quartz, while the Siberian ruby is red or pink tourmaline. The American ruby, Cape ruby, Montana ruby, and Rocky Mountain ruby are varieties of garnet.

Synthetic rubies were first produced in 1837. Improvements in the manufacturing methods of synthetic rubies, which are very much like the natural stone in physical and chemical properties, have made the synthetic gems cost effective to produce. Although synthetic rubies are used as gemstones, at least 75 percent of the annual production of synthetic rubies are used in the manufacture of watches and instruments.

Most gem scholars agree that the tradition of birthstones arose from the Breastplate of Aaron described in the Bible (Exodus 28, 15-30). The breastplate was a ceremonial religious garment set with twelve gemstones that represented the twelve tribes of Israel and corresponded with the twelve signs of the zodiac and the twelve months of the year. There are many different birthstone lists, however, and some argue that they should be assigned by astrological sign and not month. Ruby is the birthstone for July and the zodiac sign Cancer.