The mineral diamond is a crystalline form, or allotrope, of carbon (other allotropes of carbon include graphite and fullerene). It is one of the most known and most useful of more than 3,000 known minerals. Diamonds are renowned for their superlative physical qualities, especially their hardness—the word "diamond" derives from the ancient Greek adamas ("impossible to tame")—and their high dispersion of light. These properties and others make diamond valued for use in jewelry and a variety of industrial applications. Most diamonds are mined from volcanic pipes, where they have been deposited by deep-origin volcanoes drawing material from over 90 miles (150 km) deep within the Earth, where the pressure and temperature is suitable for diamond formation. Most diamonds are mined in central and southern Africa, although significant deposits have also been discovered in Canada, Russia, Brazil, and Australia. About 130 million carats (26,000 kg) of diamonds are mined annually, with a total value of nearly US$9 billion. In addition, nearly four times that mass is artificially produced as synthetic diamond.
The gemological appeal of diamonds lies in their hardness and optical properties. Diamonds used as gems are cut and polished into a number of faceted shapes in order to accentuate these attractive qualities. The hardness of diamonds allows them to hold a polish extremely well and resist scratching (only other diamonds can scratch a diamond), giving excellent luster. The dispersion of white light into a rainbow of colors, known in the trade as fire, is the other primary characteristic of gem diamonds, and has been highly prized throughout history. Gem diamonds are commonly judged by the four Cs: carat, clarity, color, and cut. Diamonds have been treasured as gems since at least 2,500 years ago, when they were used in religious icons in India. Popularity of diamonds as gemstones increased starting in the 19th century as new cutting designs that display diamonds' gem qualities better were developed.
Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools — common applications include the cutting surfaces of saw blades and drill bits, or use of diamond powder as an abrasive. Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical insulators. Industrial-grade diamonds are either unsuitable for use as gems or synthetically produced, which lowers their price and makes their use economically feasible. Industrial applications, especially as drill bits and engraving tools, also date to ancient times.
The production and distribution of diamonds is largely consolidated in the hands of a few key players, and concentrated in traditional diamond trading centers (the most important being Antwerp). The De Beers Group, based in Johannesburg, South Africa and London, England, has been the largest player in the diamond industry for over one hundred years; the company and its subsidiaries own mines that produce some 40 percent of annual world diamond production, and control distribution channels handling nearly two thirds of all gem diamonds. Some controversy over diamonds has been generated because of the monopolistic practices historically employed by De Beers including strict control of supply and alleged price manipulation, as well as the practice by some African revolutionary groups of selling conflict diamonds in order to fund their often violent activities.