Occurrence and Production
Diamonds are found in alluvial (stream-formed) deposits and in diamond pipes. Diamond pipes are tubular geological formations extending downward into the earth and composed of kimberlite or lamproite rock. Scientists believe this rock was formed far below the earth's surface, where very high pressure and temperature caused carbon to crystallize into diamond. As diamond pipes have eroded, alluvial deposits have been created.
Several hundred diamond pipes have been discovered; however, only a few can be mined profitably. The rock containing the diamond is extracted either from open pits or from underground mines, depending on the depth of the deposit. After mining, the diamond is separated from the rock by screening, crushing, and washing. Most of the commercial production of natural diamond comes from alluvial deposits. Panning is usually used to separate diamonds from gravel, sand, or other materials.
Diamonds have been found in India and Borneo since prehistoric times. In about 1720, diamonds were discovered in Brazil, which became the major producer until its supply dwindled in the early 19th century. In the 1860's diamonds were found in southern Africa, leading to the discovery of the first known diamond pipes. Diamond pipes near the present-day South African cities of Pretoria and Kimberly are famous for the large quantity of fine diamonds they have produced.
In 1986, Australia became the world's leading producer of diamonds following the discovery of a major diamond pipe and several related alluvial deposits in the western part of the country. The diamonds that have been produced are generally small and most are only of industrial quality. After Australia, other leading producers of natural diamonds include Botswana, the Democratic Republic of the Congo, Russia, and South Africa. In the United States, diamonds have been found in several widely scattered locations. One site near Murfreesboro, Arkansas, has yielded several thousand stones.
The most common process for producing synthetic diamonds involves subjecting graphite to a pressure of more than 800,000 pounds per square inch (5.5 x 109 pascals) and a temperature of 2,000 F. (1,093 C.) or greater. A low-pressure process called chemical vapor deposition (CVD) is used to coat materials with diamond film. In this process, the material to be coated is placed in a heated mixture of hydrogen and a carbon-containing gas such as methane.