A mineral composed of pure carbon. It is the hardest naturally occurring substance known. Because of their extreme hardness, diamonds have a number of important industrial applications.

The hardness, brilliance, and sparkle of diamonds make them unsurpassed as gems. Diamond stones are weighed in carats (1 carat = 200 milligrams) and in points (1 point = 0.01 carat). In addition to gem-quality stones, several varieties of industrial diamonds occur.

Diamonds are found in three types of deposits : alluvial gravels, glacial tills, and kimberlite pipes. Only in kimberlite pipes, such as those at Kimberley, S.Af., are they present in the original rock in which they were formed, probably lying at depths of more than about 75 miles (120 km). Diamonds found in alluvial and glacial gravels must have been released by fluvial or glacial erosion of the kimberlite matrix and then re-deposited in rivers or in glacial till.Diamonds vary from colourless to black, and they may be transparent, translucent, or opaque. Most industrial diamonds are gray or brown and are translucent or opaque, but better-quality industrial stones grade imperceptibly into poor quality gems. The colour of diamonds may be changed by exposure to intense radiation (as released in a nuclear reactor or by a particle accelerator) or by heat treatment.

 

A very high refractive power gives the diamond its extraordinary brilliance. A properly cut diamond will return a greater amount of light to the eye of the observer than will a gem of lesser refractive power and will thus appear more brilliant. The high dispersion gives diamonds their fire, which is caused by the separation of white light into the colours of the spectrum as it passes through the stone.

The scratch hardness of diamond is assigned the value of 10 on the Mohs scale of hardness; corundum, the mineral next to diamond in hardness, is rated as 9. Actually, diamond is very much harder than corundum; if the Mohs scale were linear, diamond's value would be about 42. The hardness of a diamond varies significantly in different directions, causing cutting and polishing of some faces to be easier than others.

In the atomic structure of diamond, as determined by X-ray diffraction techniques, each carbon atom is linked to four equidistant neighbours throughout the crystal. This close-knit, dense, strongly bonded crystal structure yields diamond properties that differ greatly from those of graphite, native carbon's other form.
  Natural Industrial Diamond
 

Any diamond that is designated for industrial use, principally as a cutting tool or abrasive. In general, industrial diamonds are too badly flawed, irregularly shaped, poorly coloured or small to be of value as gems, but they are of vital importance in the modern metalworking and mining industries. Their utility stems from the fact that diamond is the hardest substance yet known to mankind.

Industrial diamonds can be mined from natural deposits or they can be produced synthetically. Among naturally occurring diamonds, three varieties exist : ballas, boart and carbonado.

Ballas, or shot ball is composed of concentrically arranged, spherical masses of minute diamond crystals. Ballas is extremely hard, tough and difficult to cleave. Principal sources are Brazil and South America. Brazilian ballas is said to be the harder of the two.

Boart is a gray to black massive diamond, the colour of which is caused by inclusions and impurities. The name is also applied to badly coloured, flawed or irregularly shaped diamond crystals that are unsuited for gem purposes. Drilling bort is composed of small, round stones averaging 20 to the carat and is used in diamond drill bits. Crushing bort, the lowest grade of diamonds, is crushed in steel mortars and graded into abrasive grits of various sizes. 75 % of the world's crushing bort comes from Congo (Kinshasa). Its chief use is in the manufacture of grinding wheels for sharpening cement carbide, metal-cutting tools, but it also is used as loose grains suspended in oil or water for lapping and polishing.

Carbonado, known in the trade as carbon, is black, opaque diamond. It is as hard as crystallized diamond but less brittle, and, because its structure is slightly porous, it has a lower specific gravity (3.51 to 3.29). Carbonado has no cleavage and therefore is valuable for use in diamond-set tools. It usually occurs in small masses in the diamond-bearing gravels of Bahia, Brazil, and in Borneo. Rock-coring drills, widely used in exploring for new mineral deposits, are made by mounting diamonds around the rim of a hollow metal drill crown. Other important applications include saws for cutting rock and other hard materials, lathes and other types of cutting tools, glass cutters, phonograph needles, etc.

  Synthetic Industrial Diamonds
 

Man-made diamond that is usually produced by subjecting graphite to very high temperatures and pressures. Synthetic diamond resembles natural diamond in most fundamental properties, retaining the extremely hardness, broad transparency (when pure), high thermal conductivity and high electrical resistivity for which diamond is highly prized. Because synthesis is an expensive process, large stones of gem quality are rarely made. Instead, most synthetic diamond is produced as grit or small crystals that are used to provide hard coatings for industrial tools such as grinding wheels, machine tools, wire-drawing dies, quarrying saws and mining drills. In addition, diamond films can be grown on various materials by subjecting carbon-containing gas to extreme heat, these layers can be used in cutting tools, windows for optical devices or substrates for semiconductors.

In 1880, the Scottish chemist James Ballantyne Hannay claimed that he had made diamonds by heating a mixture of paraffin, bone, oil and lithium to red heat in sealed wrought-iron tubes. In 1893, the French chemist Henri Moissan announced, he had been successful in making diamonds by placing a crucible containing pure carbon and iron in an electric furnace and subjecting the very hot (about 4,000 C [7,000 F]) mixture to great pressure by sudden cooling in a water bath. Neither of these experiments has been repeated successfully.

During the first half of the 20th century the American physicist Percy Williams Bridgman conducted extensive studies of materials subjected to high pressures. His work led to the synthesis by the General Electric Company, Schenectady, N.Y., of diamonds in its laboratory in 1955. The stones were made by subjecting graphite to pressures approaching 7 gigapascals (1 million pounds per square inch) and to temperatures above 1,700 C (3,100 F) in the presence of a metal catalyst. Tons of diamonds of industrial quality have been made in variations of this process every year since 1960.

In 1961, shock-wave methods, or explosive-shock techniques, were first used to produce diamond powder, and small quantities of the material are still formed this way. Beginning in the 1950s, Russian researchers began to investigate methods for synthesizing diamond by decomposition of carbon-containing gases such as methane at high heat and low pressure. In the 1980s commercially viable versions of this chemical vapour deposition method were developed in Japan.

 

Physical Properties : Diamond is the hardest naturally occurring substance or mineral. Hardness of 10 on Moh scale. Due to crystallographic growth, diamond has harder and softer directions and can vary from 10x to 100x, thus the only two practical directions to grind diamond is parallel to a dodecahedral face or a cube face.

Density : The density or specific weight of diamond is extremely constant. The figure given is usually 3.52. A change in the Specific Gravity is as a result of impurities within the crystal. Russian scientists have shown the variation as follows : Type I -  3.51532 to 3.51542, Type II - 3.51501 to 3.5151. Lattice defects also have a bearing on density.

Thermal Conductivity : Diamond has the highest thermal conductivity of any mineral. Theoretically a red hot diamond can be dropped into liquid nitrogen without shattering.

Electrical Behaviour : Diamond is extremely good at resisting electricity. Type I stones and Type IIa stones are generally classed as extremely good insulators (a few type IIa stones have a lower resistance). Type IIb are semi- conductors. They can be used as p-type transistors (conductivity however varies from stone to stone)

 

Refractive Index : Generally given as 2.42, more accurately this is 2.4175 in sodium light.

Absorption Spectra : There exists over 100 absorption lines found in diamond spectra; they also migrate with reduced temperatures to their more correct positions. The most predominant types of diamonds are: Type Ia 'Cape series' and the main line that is good to remember is the 415.5 absorption line. For most absorption spectra to be  seen inc 415.5 line, sophisticated instrumentation such as a spectrophotometer at the temperature of liquid nitrogen need to be used for easier identification, so that rules out the trusty hand held spectroscope.

Ultra-violet reactions : The most common fluorescence is blue, ranging from bright sky blue to weak dull violet. Intensity varies considerably. Other colours of fluorescence is green, orange and very rarely, pink or red. Blue fluorescencing diamonds will phosphoresce in shades of yellow or green, but this is usually hard to see.


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