Industrial diamonds

Diamond is one of the world’s most important minerals and gemstones; it is the element carbon (C) crystallized in the isometric system.

Background

Diamond is the hardest known substance, natural or artificial, and is number 10 on the Mohs scale. The close-packed cubic arrangement of the atoms gives diamond its unique hardness. It also has the highest thermal conductivity of any known substance.

Historical records of diamonds date back to 3000 BCE. In recent centuries, Golconda diamonds of India dominated diamond production until the early eighteenth century. In 1725, Brazilian diamond mines gained prominence. South Africa’s “great diamond rush” began in 1867, and in 1890, the De Beers company consolidated dozens of mining communities in Africa. Diamond derives its name from the Greek word adamas, which means “unconquerable.”

Almost all the world’s diamond production comes from Africa, most notably South Africa. Other diamond-producing countries include Angola, Borneo, Ghana, Guyana, Namibia, Sierra Leone, Tanzania, Venezuela, Congo, Brazil, and Russia (Siberia). In the United States, diamonds have been found in Arizona, Arkansas, Montana, and Nevada.

Technical Definition

The atomic number of carbon is 6, and its atomic weight is 12.011. It belongs to Group IVA of the periodic table of elements. Gem diamonds have a of 3.52, although “black diamonds” have a density of about 3.15. Diamond slowly burns to carbon dioxide at a very low temperature (900° Celsius).

Diamond’s high refractive index (2.417) and strong dispersion property (0.058) guarantee its supremacy as a gemstone. However, only about one-fifth of all the diamonds mined qualify as gems. Most of the remaining uncut diamonds are used by industry. Tunnel boring and oil-well drilling equipment uses diamond-studded rotary bits. Carbide grinding wheels, abrasion-resistant cutting tools, and glass-etching and glass-cutting equipment use industrial-quality diamonds. Some dentists and surgeons use diamond-headed scalpels to cut delicate bones and tissue. Diamond coatings are used in integrated circuits, prosthetic devices, and biosensors. Diamond is the most important industrial abrasive, and industry uses about 80 percent (by weight) of all diamonds produced. However, this represents only about 30 percent by value.

Creation and Properties of Diamond

About 30 meters inside the Earth, exceedingly high pressures and temperatures (more than 1,400° Celsius) cause magnesium-rich melts to crystallize, resulting in the formation of diamonds. Samples of deep material contain diamonds as a natural component. The reaction of with hot, magnesium-rich, deep mantle material aided by carbon dioxide leads to the formation of a rock called kimberlite. Kimberlite is an rock that is ultrabasic and contains very little silica. Kimberlite is the world’s principal source of diamonds. Explosive eruptions create craters filled with deep mantle rock formations and permit diamond-containing rocks to surface through cracks. These are known as diamond pipes (sometimes incorrectly called “volcanic necks”).

In addition to being the hardest substance, diamond is an excellent conductor of heat. Because diamonds possess the highest thermal conductivity of any known substance, industrial-quality diamonds are used in abrasion-resistant cutting tools. Almost all diamonds are nonconductors of electricity. However, some diamonds permit the passage of electric current when bombarded with radiation. Diamond crystals form as octahedrons, dodecahedrons, and cubes. A well cut gem can reflect almost all the light that it receives. This quality is called “luster.” In addition, it can disperse or separate the colors of the spectrum while reflecting the incident light. This quality is called “fire.”

Occurrence of Diamond

Mining experts have discovered hundreds of diamond-containing dikes and pipes in Transvaal, Kimberley District, and Free State (formerly Orange Free State), South Africa; Yakutsk, Siberia; Shinyanga, Tanzania; Mbuji-Mayi, Democratic Republic of the Congo; Yengema, Sierra Leone; Murfreesboro, Arkansas; and several other locations. However, it is not economically feasible to carry out “mine-at-depth” procedures in most of these pipe mines. Unless new pipes are discovered, natural diamonds may be exhausted relatively soon.

The of diamond pipes over millions of years has resulted in secondary deposits called alluvial or placer deposits. These deposits contribute significantly to the world’s total diamond production. Most alluvial diamonds are recovered from stream gravel, but beach gravel is also a good source. Diamond-containing beach gravel extends to the depths of the ocean floor, although there is no economical means of recovering diamonds from ocean depths. Diamonds are also found in glacial tills. Minute quantities of microscopic diamonds have been found in meteorites as well.

Synthetic Diamonds

On February 15, 1955, the General Electric Company announced its success in creating a synthetic diamond. Since then, synthetic diamonds have become widely used in grinding wheels and a number of other applications. They are normally single crystals, usually octahedral in shape. Since they have several cutting edges, they are preferred over natural diamonds for industrial purposes. To make them, graphite (another form of crystalline carbon) is subjected to very high temperatures and pressures. Extreme pressures as high as 296,076 atmospheres (about 30 billion pascals) and temperatures as high as 3,037° Celsius (water boils at 100° Celsius) have been used, depending upon the actual process. Two common procedures are shock conversion and static conversion. Synthetic diamonds are also manufactured by the static of certain alloys and molten metals.

Synthetic diamonds, normally black in color, are produced in grain sizes that are about one-hundredths of a centimeter in diameter. It is possible to “grow” larger, gem-quality synthetic diamonds, but the process is too costly to be feasible. Synthetic diamonds are chemically and crystallographically identical to the naturally occurring diamond gemstone. “Imitation diamonds,” on the other hand, are completely different from either synthetic or genuine diamonds. Imitation diamonds do not possess either the hardness or the crystallographic structure of the genuine diamond; they are chemically different. They are made of glass or other materials and are simply intended to imitate the appearance of a diamond.

Cutting Diamond

After mining and recovery, gem-quality diamonds are separated from industrial-quality ones. A rough, uncut diamond looks like a dull piece of glass. Precise cutting, artful grinding, and skillful polishing of the diamonds yield outstanding gems, and some have attained historic fame. Diamond cutting began in India and was later perfected in Italy. Only a diamond can cut a diamond: Diamond crystals are cut, cleaved, shaped, and polished by “diamond dust on a lap.” World-famous diamond cutting establishments are concentrated in Antwerp, Belgium, and in Amsterdam, the Netherlands. India and Israel have also emerged as world leaders in diamond cutting. The most popular cut is the “brilliant cut,” which has a round shape with fifty-eight facets. Gem-quality diamonds are classified according to their weight, clarity, color, and absence of flaws. The weight of a diamond is measured in carats; a carat equals 0.2 gram, or about 0.00704 ounce. Transparent, colorless, and light blue diamonds are extremely rare and are considered to be highly valuable gems. There are red, pink, blue, and green diamonds. Diamonds with a yellow tint are more common. As the tint becomes increasingly yellowish, the value decreases. Industrial-quality diamonds are gray, brown, or black and are almost opaque. They are gems of poor quality.

Famous Diamonds

The largest diamond ever discovered, the Cullinan, was found in 1905 in the Premier Mine, Transvaal, South Africa, and weighed 3,106 carats. This was cut and polished into several gems, two of them world famous: The 530-carat Star of Africa and the 309-carat Star of Africa II are among the British crown jewels, housed in the Tower of London. These are the world’s largest cut diamonds. The cutting of the Cullinan also resulted in another seven large gems and ninety smaller ones.

The 109-carat Koh-i-Noor (“mountain of light”), set in the British crown itself, is the oldest diamond gemstone known to historians; its history has been traced back to 1304. This diamond had its origin in India, and it originally weighed 186 carats before Queen Victoria had it recut in 1852. Many believe that the largest blue diamond, the 44.5-carat Hope diamond, presently in the Smithsonian Institution, adorned the eye of an Indian god. Other world-famous diamonds that India has contributed include the Regent or Pitt (140 carats, presently in the Louvre, France); the Orlov (200 carats, presently in Russia); the Florentine (137 carats, location unknown); and the Great Mogul (280 carats, location also unknown).

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