Introduction to Diamond
Diamond, a hard, crystalline form of carbon. Diamonds are valued both for their beauty as gems and for their industrial uses. They are typically colorless, but may occur in tints of blue, green, black, brown, light yellow, orange, or pink. Most diamonds have an eight-sided crystalline structure. The diamond is the birthstone for April, and is a symbol of innocence.
Diamond is the hardest natural substance known, and it will not dissolve in any known solvent. It conducts heat much better than any other material, including metals. When ignited in oxygen at a temperature of 1,600 F. (870 C.) or more, it will burn.
Diamonds occur in many parts of the world and are also produced synthetically. Synthetic diamonds have been produced since the 1950's. They are virtually identical to natural diamonds, but, because of the expense of their manufacture, are usually produced only in small sizes suitable for industrial use. In the 1980's, a process was developed for producing a very thin film of diamond on other materials.
Diamonds are weighed by the carat. One carat is equal in weight to 200 milligrams, or approximately 0.007 ounce.
Uses of Diamonds
Diamonds are valued as gems because they are rare and because they refract (bend) light to a greater degree than any other gem.
This refraction causes light rays entering the diamond to be broken up into many colors. Diamonds that are very small, structurally defective, or otherwise unsuitable for gemstones are used as industrial diamonds. Industrial diamonds are important for cutting, grinding, polishing, and scraping various kinds of materials. They are also used as dies for making wire. Small diamond fragments are used to provide cutting edges on such tools such as earth drills and rock-cutting saws. Diamond films are used commercially on cutting tools to form a wear-resistant coating and on microelectronic chips as heat sinks to absorb unwanted heat.
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.
The Diamond Industry
The De Beers Central Selling Organization controls most of the world's supply of uncut diamonds through an international marketing cartel. Most of the world's producers of diamonds belong to the cartel to help maintain a high price for diamonds. De Beers companies have controlled the supply of diamonds since the late 1880's. Major centers for the cutting and selling of diamonds include New York City; Amsterdam, Netherlands; Antwerp, Belgium; Tel Aviv, Israel; and several cities in India.
The cutting and polishing of diamonds is called dressing. One of two processes is used: sawing or cleaving. Most diamonds are sawed. The rough stone is cut to a desired shape by a very thin, rapidly revolving disc the edges of which are embedded with diamond dust mixed with olive oil. In cleaving, a steel blade is placed against a natural groove in the diamond and struck a hard, sharp blow. The stone cracks along the line of the groove, much as wood will crack along the line of its grain. The faces of the split stone are then polished by whirling steel discs coated with diamond dust and olive oil.
Diamonds may be sawed or cleaved into many different shapes. The top of the diamond usually forms a fairly large flat surface, but the sides and bottom are generally cut into dozens of small, flat surfaces called facets. Light entering through one facet is reflected out through another. The refracting quality of the gem makes the light scatter, causing the diamond to give off a sparkling, many-colored light. This effect is increased by increasing the number of facets.
The value of a gem diamond depends upon its weight, the way in which it is cut, its color, and its purity. A pure diamond is one without flaws—that is, without cracks, pores, or deposits of either foreign material or uncrystallized carbon.