Diamonds

Diamond is an important industrial mineral as well as the most valued gemstone. Natural diamonds crystallize only at very high pressures and are brought to the Earth’s surface in kimberlite, an unusual type of igneous rock that forms in the upper mantle. Kimberlite also contains "gems" of another sort: rare pieces of the Earth’s deep crust and mantle.

Availability of Diamonds

In the eighteenth century, virtually all the world’s diamonds came from India and were hoarded by royalty; few had ever seen a diamond, let alone possessed one. Brazil became an important producer in the late 1700s, but diamonds were still unavailable to most people. In 1866, a farm boy discovered a bright pebble on the banks of the Orange River in South Africa and unknowingly started a chain of economic, social, and political upheavals that continue to this day; the stone was later determined to be a 21-carat diamond (one carat equals 200 milligrams). Within a decade, South Africa’s mines would be producing 3 million carats a year for a world market; by the turn of the century, diamonds would become an important industrial commodity.

Although South Africa remained among the world’s foremost locations for diamond mining, Russia was the world’s largest producer in 2023 with 37.32 million carats of diamonds. The African nation of Botswana was second with 25.09 million carats. In the 2020s, all diamond mines in North America were concentrated in Canada, which was the third-largest diamond producer at 15.98 million carats. A few diamond mines once operated in the United States—most notably in Arkansas and Colorado—but the mines were closed prior to 2002.

Early diggings were concentrated along the Orange and Vaal rivers of South Africa, where prospectors staked small claims and shoveled into the diamondiferous gravel deposits. Fines were washed away in a rudimentary method known as wet digging, and the remaining gravel was spread out to be examined, pebble by pebble. Around 1870, a diamond was found approximately 100 kilometers from the nearest river, and prospectors began to speculate that the stones along the riverbanks did not originate there but were washed in from elsewhere. Increasing numbers of miners went into the bush to pursue diamonds, finding them in local patches of weathered rock known as yellow ground. As geologists would soon understand, the yellow ground was merely the uppermost layer of deep, funnel-shaped pipes of diamond-bearing igneous rock that had been injected into the earth’s crust by ancient volcanoes. The bluish-gray rock was named kimberlite for a nearby South African town. Diamonds are also mined from alluvial deposits in Russia and Namibia.

In some African nations, civil wars have been financed by the illicit mining and sale of diamonds, called "conflict diamonds" or "blood diamonds." Frequently, the diamonds are mined by slave labor. A series of international conventions requiring the registration of diamonds and strict controls on shipments has significantly curtailed the trade in conflict diamonds.

Inclusion in Kimberlite

More than 90 percent of the world’s diamonds are found in river gravel, beach sands, and glacial deposits of many geological ages. Only in kimberlite pipes are diamonds found in the original rock in which they were formed. Kimberlite pipes have rather inconspicuous features, seldom having diameters greater than 1 kilometer. Mining has shown them to be carrotlike bodies whose vertical dimensions far exceed the sizes of their surface outcrops. Mine shafts have penetrated about 1.5 kilometers into kimberlite pipes, but minerals contained in the kimberlite, including the coveted diamonds, suggest that the pipes extend all the way through the crust and into the earth’s upper mantle to a total depth of about 250 kilometers. This is deeper than any other variety of igneous rock. Most kimberlites do not contain commercial quantities of diamonds.

Beneath the weathered yellow ground, fresh kimberlite is a hard, dark bluish-gray rock that miners call blue ground. Its texture gives strong evidence of an igneous origin, indicating that kimberlite was injected into the earth’s crust as a molten liquid and then quickly solidified against cooler rocks surrounding the pipe. The major constituents of kimberlite are silicate minerals: compounds of silicon and oxygen with other metal ions. Kimberlite is a variety of peridotite ("peridot" is an ancient word for olivine), and hence its major constituent is the mineral olivine, a magnesium-iron silicate. The olivine is usually altered to the mineral serpentine, giving the rock its characteristic blue-gray color.

Exotic rocks contained within the kimberlite matrix are perhaps more interesting than the kimberlite itself. Diamonds are one such inclusion, although they constitute a minuscule proportion of the total rock. Typical diamond contents in minable kimberlite range from about 0.1 to 0.35 carat per ton; even the famous Premier Mine in South Africa has produced only about 5.5 tons of diamonds from 100 million tons of rock, which is about 0.000005 percent.

Formation of Diamonds

Three independent lines of evidence indicate that kimberlite is formed in the upper mantle at depths and pressures far greater than any other type of igneous rock. To calculate the pressures of formation, it is usually necessary to know or assume the temperature of formation. The continental geotherm supplies the needed temperature information. At temperatures along the continental geotherm, diamond is stable only at depths greater than about 100 kilometers; at lower pressures, graphite forms instead. Diamond exists at low pressures only because it is metastable (otherwise, there could be no diamonds on the earth’s surface), but diamond does not form naturally at low pressures.

A second line of evidence for great pressure involves the minerals that have been trapped inside the diamonds during their growth. Diamonds sometimes contain inclusions of coesite, a mineral with the same chemical composition as quartz (silicon dioxide) but with a more compact structure that forms only at very high pressures. At temperatures along the continental geotherm, laboratory studies have shown that coesite, in turn, gives way to the silica mineral stishovite at depths greater than about 300 kilometers. Because stishovite has never been found in diamonds, the diamonds must have formed at depths of less than 300 kilometers or so. Hence, diamonds seem to have formed at depths between about 100 and 300 kilometers in the earth’s upper mantle. The pressures and temperatures that have been calculated from minerals in the ultramafic nodules are in agreement with this depth range: Most of the nodules seem to have formed at depths of 100 to 250 kilometers in the upper mantle and at temperatures of about 1,100 to 1,500 degrees Celsius. Contrary to popular misconception, diamond never forms from the metamorphism of coal. Surface rocks are never buried at the depths where diamonds form, and the carbon of diamonds has been in the mantle since the formation of the earth.

Physical Properties

The unusual physical properties of a diamond are a reflection of its crystalline structure. Diamond is a three-dimensional network of elemental carbon, with each carbon atom linked to four equidistant neighbors by strong covalent bonds (in which electrons are equally shared by two atoms). The dense, strongly bonded crystal structure gives diamond its extreme hardness. Another mineral made of pure carbon is graphite, the writing material in lead pencils. In graphite, sheets of carbon atoms are weakly bonded and separated by relatively large distances. Thus, graphite has a lamellar structure and is very soft.

The physical properties of diamonds are remarkable in comparison to virtually all other materials. It is the hardest substance known. With a hardness of 10, it tops the Mohs scale of relative hardness and is actually about forty-two times as hard as corundum, its nearest neighbor, with a hardness of 9. Its luster falls between adamantine and greasy; it cannot be wetted by water, a property that is of great practical benefit in separating diamonds from waste rock. Diamonds vary in color from water-clear (designating the most valuable diamonds) to pale blue, yellow, deep yellow, or brown; industrial varieties are brown or grayish-black. Raw diamonds occur most often as octahedra (eight-sided polygons) or irregular shapes. Diamond can be cleaved in four directions parallel to its octahedral faces. Its refractive index of 2.42 is the highest of all gems, producing strong reflections in cut stones, and its very high dispersion (that is, its ability to separate white light into the colors of the spectrum) gives cut diamonds their "fire." Diamond is also triboelectric (electrically charged when rubbed) and fluorescent (emitting visible light when struck by ultraviolet rays).

Diamonds are separated from waste rock by first crushing the kimberlite, wetting it, then passing it over a series of greased bronze tables. Diamond cannot be wetted by water and is the only mineral that sticks to the grease, which is later scraped off as the diamonds are extracted. Another way of separating diamonds takes advantage of their fluorescent property. As the crushed rock is passed beneath ultraviolet lamps, the diamonds are spotted by photosensors, which trigger jets of compressed air that eject the diamonds into bins. Gem-quality stones of less than 1 carat are called melee. Less than 5 percent of all diamonds are suitable for cut stones of 1 carat or larger. Not all diamonds are gem quality. Black or dark diamonds full of graphite inclusions are called "carbonado"; these are suitable only for use as abrasives.

Diamond Cutting and Grading

Diamonds have been fashioned into precious jewels for several millennia, but diamond cutting has become a major industry only during the past century in response to worldwide demand and the abundant supply of South African diamonds. Five basic steps are involved in diamond cutting: marking, cleaving, sawing, girdling, and faceting. The diamond is first carefully studied, sometimes for months, in order to identify its cleavage planes and to map out any inclusion-rich areas that will affect how it is to be cut. Large diamonds are usually irregular in shape and are seldom left whole. A central master stone is commonly envisioned within the mass, and the "satellite" offcuts become fine gems in their own right. Lines for cleaving or sawing are marked in black ink, and the diamond is then sent to the cutter.

If the diamond is to be cleaved, a thin groove is first established using a saw charged with diamond dust. The diamond is mounted in a dop or clamp, and a steel wedge is inserted into the groove and struck sharply with a mallet. A misdirected blow can shatter the stone. It is said that Joseph Asscher, after successfully cleaving the 3,106-carat Cullinan diamond in 1908, swooned into the arms of an attending physician. Sawing is a slower alternative to cleaving; it uses a thin, circular bronze blade that is charged with diamond dust (diamonds differ in hardness in different directions in the crystal; the cutting is done by grains that happen to have suitable orientations). Next, the diamond is girdled by placing it in a lathe and grinding it against another diamond to make it round; the "girdle" is where the upper and lower sets of facets meet. Finally, the diamond is faceted: The facets are cut and polished by clamping the stone in a holder and placing it against rotating laps that are charged with diamond dust. The most popular cut is the "brilliant"—a round stone with fifty-eight facets. Other cuts include the marquise (oval), emerald (rectangular), and pear. The orientations of the facets are calculated carefully to take advantage of the optical properties of the diamond.

Cut stones are graded under strict rules, using an elaborate system of four criteria: cut, color, clarity, and carat. Cut refers to the skill with which the gem has been shaped—its symmetry and reflective brilliance. Color refers to the tint of the stone: The most valuable gems are water-clear, but many fine stones are pale yellow, blue, or pink; colored diamonds are called "fancies." Clarity refers to the size, number, and locations of any inclusions that may be present. Inclusions do not necessarily degrade a stone if they are small or located in inconspicuous places. Carat refers to the weight of the stone. The largest diamond ever discovered was the Cullinan diamond, unearthed in South Africa in 1905, which was measured at 3,106 carats (1.37 pounds, or 0.62 kilograms) before it was cleaved. The Cullinan produced nine large stones—including the 530-carat Cullinan I (also known as the Star of Africa) and the 317-carat Cullinan II, the world's two largest cut clear diamonds—and ninety-six smaller ones. The nine large stones were either retained by or later presented to the British royal family, with the two largest becoming centerpieces of the crown jewels, while the smaller ones were given to Asscher, the cutter, for his fee.

Study of Diamonds

Diamonds have been studied using the same analytical methods that are applied to other crystalline solids—notably X-ray diffraction—to identify their crystalline structure. Mineral inclusions in diamond (mostly coesite and garnet) have been examined with the electron microprobe, a device that employs a tiny electron beam to measure the percentages of elements that are present in a mineral. The compositions of coexisting minerals in kimberlite and ultramafic nodules have been similarly analyzed.

Igneous petrology is a subdiscipline of geology concerned with the description and origin of igneous rocks. Petrologists use all the methods listed above for individual minerals, plus larger-scale observations of entire rock masses. Minerals are assemblages of atoms, and rocks are assemblages of minerals; hence, understanding the origins of minerals, including diamond, involves an understanding of how the enclosing rock was formed. Individual rock masses such as kimberlite, ultramafic nodules, and xenoliths of the crust are glued onto glass slides, ground into thin slices, and studied under the microscope. In addition to the minerals they contain, the textures of rocks reveal much about their origins. The results of mineralogical and textural observations are then interpreted in light of even larger-scale observations involving geological mapping on the surface and underground. Finally, all mineralogical, petrological, and geological observations must be interpreted within the constraints of geophysical data on the internal constitution of the earth.

The quest for diamonds has fostered much inquiry into the rock in which diamonds are found. Kimberlite is a very unusual type of igneous rock that forms deep in the Earth’s mantle, at depths up to 250 kilometers. The ultimate source of diamonds, kimberlite also contains "gems" of a different sort: pieces of the Earth’s deep continental crust and upper mantle that would be inaccessible by any other means. Volcanic pipes of kimberlite are, therefore, windows into the Earth’s interior, and the rock and its inclusions are avidly studied in order to learn more about the internal constitution of the Earth. Diamonds contain trapped inclusions of liquids, mineral solids, and gases, mostly carbon dioxide. Analyses of these inclusions have led to a better understanding of the conditions under which diamond is formed and the volatiles that are present in the Earth’s mantle.

Industrial Uses

The hardness, brilliance, and fire of diamonds have made them unsurpassed as gems; the use of diamonds as industrial materials is perhaps less well-known. All stones not suitable for use as gems are destined for industrial use. "Bort" refers to dense, hard, industrial diamonds, and carbonado refers to diamond that has a lower specific gravity than normal diamond. Such diamonds vary in color from off-white to black. Most industrial diamonds are used as abrasives. Crushed into various sizes, they are used for grinding wheels, grinding powders, polishing disks, drill bits, and saws. Diamond is indispensable for grinding the tungsten carbide cutting tools that have been in use since the 1930s. Industrial diamonds also are sorted by shape: Blocky stones are suited for more severe grinding operations, such as rock-drill bits, and more splintery ones are reserved for grinding tungsten carbide. Gem diamonds are pretty, but industrial diamonds are absolutely essential to modern technology.

In 1955, the General Electric Company succeeded in synthesizing diamonds at low pressure. Initially more expensive than natural stones, synthetic diamonds are now widely used in grinding wheels to sharpen tungsten carbide. Synthetic diamonds are smaller but have rougher surfaces than natural stones and are manufactured in a variety of shapes and sizes for specialized abrasive applications.

Diamonds are also used as dies for drawing out the fine tungsten filaments of incandescent light bulbs, as scalpels for eye surgery, and as stereo phonograph needles. Unrivaled as a heat conductor, diamond is an important component in the miniature diodes that are used in telecommunications. Diamond has even served as a tiny instrument window on the Pioneer space probe to Venus, as it tolerates the extremes of heat and cold in outer space. Another application of diamonds is the diamond-anvil pressure cell. Two gem-quality diamonds are used to subject samples to enormous pressure, duplicating conditions deep in the earth. The diamond windows allow the samples to be observed under a microscope during experimentation. This device, which fits in the palm of one’s hand, can perform experiments that once required massive and dangerous hydraulic presses.

Diamonds have long been synonymous with marriage in modern Western society. Although society is moving away from this trend in the twenty-first century, diamonds still retain their utility in other industries.

Principal Terms

bort: a general term for diamonds that are suitable only for industrial purposes; these diamonds are black, dark gray, brown, or green in color and usually contain many inclusions of other minerals

coesite: a mineral with the same composition as quartz (silicon dioxide) but with a dense crystal structure that forms only under very high pressures

crystal: a solid that possesses a definite orderly arrangement of its atoms; crystal differs from an amorphous solid such as glass, and all true minerals are crystalline solids

graphite: a crystalline variety of the element carbon, characterized by its softness and ability to cleave into flakes; the carbon atoms are arranged in sheets that are weakly bonded together

kimberlite: an unusual, fine-grained variety of peridotite that contains trace amounts of diamond

metastable: the state of crystalline solids once they are outside of the temperature and pressure conditions under which they formed; thus, diamond forms at very high pressures within the Earth but is metastable at the Earth’s surface

peridotite: a dense, dark-green rock that is composed mainly of magnesium- and iron-rich silicates, particularly olivine; the mantle, and the ultramafic nodules derived from it, is composed of peridotite

ultramafic: rocks such as peridotite that contain abundant magnesium- and iron-rich minerals

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