Diamond

Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond as a form of carbon is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of electricity, and insoluble in water. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, but diamond is metastable and converts to it at a negligible rate under those conditions. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are used in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions are boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) can color a diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange, or red. Diamond also has a very high refractive index and a relatively high optical dispersion.

Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometres (93 and 155 mi) in the Earth's mantle, although a few have come from as deep as 800 kilometres (500 mi). Under high pressure and temperature, carbon-containing fluids dissolved various minerals and replaced them with diamonds. Much more recently (hundreds to tens of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.

Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gases by chemical vapor deposition (CVD). Natural and synthetic diamonds are most commonly distinguished using optical techniques or thermal conductivity measurements. 

Diamond

General

Category Native minerals

Formula C

IMA symbol Dia

Strunz classification 1.CB.10a

Dana classification 1.3.6.1

Crystal system Cubic

Crystal class Hexoctahedral (m3m)

H-M symbol: (4/m 3 2/m)

Space group Fd3m (No. 227)

Structure

Jmol (3D) Interactive image

Identification

Formula mass 12.01 g/mol

Color Typically yellow, brown, or gray to colorless. Less often blue, green, black, translucent white, pink, violet, orange, purple, and red.

Crystal habit Octahedral

Twinning Spinel law common (yielding "macle")

Cleavage 111 (perfect in four directions)

Fracture Irregular/Uneven

Mohs scale hardness 10 (defining mineral)

Luster Adamantine

Streak Colorless

Diaphaneity Transparent to subtransparent to translucent

Specific gravity 3.52±0.01

Density 3.5–3.53 g/cm3 3500–3530 kg/m3

Polish luster Adamantine

Optical properties Isotropic

Refractive index 2.418 (at 500 nm)

Birefringence None

Pleochroism None

Dispersion 0.044

Melting point Pressure dependent

Etymology, earliest use and composition discovery

The name diamond is derived from Ancient Greek: ἀδάμας (adámas), 'proper, unalterable, unbreakable, untamed', from ἀ- (a-), 'not' + Ancient Greek: δαμάω (damáō), 'to overpower, tame'. Diamonds are thought to have been first recognized and mined in India, where significant alluvial deposits of the stone could be found many centuries ago along the rivers Penner, Krishna, and Godavari. Diamonds have been known in India for at least 3,000 years but most likely 6,000 years.

Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history. The popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns.

In 1772, the French scientist Antoine Lavoisier used a lens to concentrate the rays of the sun on a diamond in an atmosphere of oxygen, and showed that the only product of the combustion was carbon dioxide, proving that diamond is composed of carbon. Later, in 1797, the English chemist Smithson Tennant repeated and expanded that experiment. By demonstrating that burning diamond and graphite releases the same amount of gas, he established the chemical equivalence of these substances.

Classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz, the diamond belonged to the mineral class of the "elements" and there to the department of "semimetals and non-metals", where it formed an independent group together with chaoite, fullerite, graphite, lonsdaleite and moissanite.

The 9th edition of Strunz's mineral classification, valid since 2001 and used by the International Mineralogical Association (IMA), also classifies diamonds in the class of "elements" and within the division of "semimetals (metalloids) and nonmetals." However, this division is further subdivided according to related chemical elements, so the mineral is found in the subdivision "carbon-silicon family," where it forms the unnamed group 1.CB.10 together with lonsdaleite.

The Dana classification of minerals also classifies diamonds in the class and division of the same name, "Elements." Here, it is found together with graphite, lonsdaleite, chaoite, and fullerite in the "Carbon Polymorphs" with the system number 01.03.06 within the subdivision " Elements: Semimetals and Nonmetals."

Properties

Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common allotropes of pure carbon are diamond and graphite. In graphite, the bonds are sp2 orbital hybrids and the atoms form in planes, with each bound to three nearest neighbors, 120 degrees apart. In diamond, they are sp3 and the atoms form tetrahedra, with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, and, of all known substances, diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least compressible. It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m3 in pure diamond. In graphite, the bonds between nearest neighbors are even stronger, but the bonds between parallel adjacent planes are weak, so the planes easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable.

Diamonds have been adopted for many uses because of the material's exceptional physical characteristics. It has the highest thermal conductivity and the highest sound velocity. It has low adhesion and friction, and its coefficient of thermal expansion is extremely low. Its optical transparency extends from the far infrared to the deep ultraviolet and it has high optical dispersion. It also has high electrical resistance. It is chemically inert, not reacting with most corrosive substances, and has excellent biological compatibility.

Thermodynamics

The equilibrium pressure and temperature conditions for a transition between graphite and diamond are well established theoretically and experimentally. The equilibrium pressure varies linearly with temperature, between 1.7 GPa at 0 K and 12 GPa at 5000 K (the diamond/graphite/liquid triple point). However, the phases have a wide region about this line where they can coexist. At standard temperature and pressure, 20 °C (293 K) and 1 standard atmosphere (0.10 MPa), the stable phase of carbon is graphite, but diamond is metastable, with a significant kinetic energy barrier that the atoms must overcome in order to reach the lower energy state, and its rate of conversion to graphite is negligible, with a timescale of millions to billions of years. However, at temperatures above about 4500 K, diamond rapidly converts to graphite. Experiments have found that diamond in the presence of H2O passes through an intermediate linear carbon phase.

Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa (about 350,000 standard atmospheres) is needed.

Above the graphite–diamond–liquid carbon triple point, the melting point of diamond increases slowly with increasing pressure; but at pressures of hundreds of GPa, it decreases. At high pressures, silicon and germanium have a BC8 body-centered cubic crystal structure, and a similar structure is predicted for carbon at high pressures. At 0 K, the transition is predicted to occur at 1100 GPa.

Results published in Nature Physics in 2010 suggest that, at ultra-high pressures and temperatures (about 10 million atmospheres or 1 TPa and 50,000 °C), diamond melts into a metallic fluid. The extreme conditions required for this to occur are present in the ice giant planets Neptune and Uranus, both of which are made up of approximately 10 percent carbon and could hypothetically contain oceans of liquid carbon. Since large quantities of metallic fluid can affect the magnetic field, this could serve to explain why the geographic and magnetic poles of the two planets are not aligned.

Crystal structure

The most common crystal structure of diamond is called diamond cubic. It is formed of unit cells (see the figure) stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell. The length of each side of the unit cell is denoted by a and is 3.567 angstroms.

The nearest neighbor distance in the diamond lattice is 1.732a/4 where a is the lattice constant, usually given in Angstrøms as a = 3.567 Å, which is 0.3567 nm.

A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1⁄4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point. Viewed from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC... pattern. Diamonds can also form an ABAB... structure, which is known as hexagonal diamond or lonsdaleite, but this is far less common and is formed under different conditions from cubic carbon.

Crystal habit

Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. As diamond's crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, octahedron, rhombicosidodecahedron, tetrakis hexahedron, or disdyakis dodecahedron. The crystals can have rounded-off and unexpressive edges and can be elongated. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin.

Some diamonds contain opaque fibers. They are referred to as opaque if the fibers grow from a clear substrate or fibrous if they occupy the entire crystal. Their colors range from yellow to green or gray, sometimes with cloud-like white to gray impurities. Their most common shape is cuboidal, but they can also form octahedra, dodecahedra, macles, or combined shapes. The structure is the result of numerous impurities with sizes between 1 and 5 microns. These diamonds probably formed in kimberlite magma and sampled the volatiles.

Diamonds can also form polycrystalline aggregates. There have been attempts to classify them into groups with names such as boart, ballas, stewartite, and framesite, but there is no widely accepted set of criteria. Carbonado, a type in which the diamond grains were sintered (fused without melting by the application of heat and pressure), is black in color and tougher than single crystal diamond. It has never been observed in a volcanic rock. There are many theories for its origin, including formation in a star, but no consensus.

Mechanical

Diamond oxidizes in pure oxygen at around 720 °C, and in air at a slower reaction rate above 720 °C to (gaseous) carbon dioxide. If a diamond of a few millimeters in diameter, heated to yellow heat, is placed in liquid, i.e., cryogenic oxygen, it sinks and burns with a glowing appearance to form carbon dioxide, which precipitates as a solid. The reaction enthalpy of 395.7 kJ/mol is 1.89 kJ/mol greater than that of graphite. To burn diamond in a gas flame, an excess of oxygen is needed. Diamond powder with a suitable grain size of around 50 micrometers burns in a shower of sparks after contact with a flame, similar to coal powder. Accordingly, pyrotechnic compositions can also be made from diamond powder. While the spark color is comparable to coal powder, a very linear trajectory is observed due to the approximately twice higher density. It reacts with hydrogen at high temperatures to form hydrocarbons.

Diamond dissolves in melts of carbon-dissolving metals such as iron, nickel, cobalt, chromium, titanium, platinum, palladium, and their alloys. The larger the grain or crystal, the lower the reaction rate—in all cases, according to the ratio of reactive surface area to volume.

Hardness

Diamond is the hardest material on the qualitative Mohs scale. To conduct the quantitative Vickers hardness test, samples of materials are struck with a pyramid of standardized dimensions using a known force – a diamond crystal is used for the pyramid to permit a wide range of materials to be tested. From the size of the resulting indentation, a Vickers hardness value for the material can be determined. Diamond's great hardness relative to other materials has been known since antiquity, and is the source of its name. This does not mean that it is infinitely hard, indestructible, or unscratchable. Indeed, diamonds can be scratched by other diamonds and worn down over time even by softer materials, such as vinyl phonograph records.

Diamond hardness depends on its purity, crystalline perfection, and orientation: hardness is higher for flawless, pure crystals oriented to the <111> direction (along the longest diagonal of the cubic diamond lattice). Therefore, whereas it might be possible to scratch some diamonds with other materials, such as boron nitride, the hardest diamonds can only be scratched by other diamonds and nanocrystalline diamond aggregates.

The hardness of diamond contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in engagement or wedding rings, which are often worn every day.

The hardest natural diamonds mostly originate from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is associated with the crystal growth form, which is single-stage crystal growth. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice, all of which affect their hardness. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds that are harder than the diamonds used in hardness gauges.

Diamonds cut glass, but this does not positively identify a diamond because other materials, such as quartz, also lie above glass on the Mohs scale and can also cut it. Diamonds can scratch other diamonds, but this can result in damage to one or both stones. Hardness tests are infrequently used in practical gemology because of their potentially destructive nature. The extreme hardness and high value of diamond means that gems are typically polished slowly, using painstaking traditional techniques and greater attention to detail than is the case with most other gemstones; these tend to result in extremely flat, highly polished facets with exceptionally sharp facet edges. Diamonds also possess an extremely high refractive index and fairly high dispersion. Taken together, these factors affect the overall appearance of a polished diamond and most diamantaires still rely upon skilled use of a loupe (magnifying glass) to identify diamonds "by eye".

Toughness

Somewhat related to hardness is another mechanical property toughness, which is a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 50–65 MPa•m1/2.[contradictory] This value is good compared to other ceramic materials, but poor compared to most engineering materials such as engineering alloys, which typically exhibit toughness over 80 MPa•m1/2. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a cleavage plane and is therefore more fragile in some orientations than others. Diamond cutters use this attribute to cleave some stones before faceting them. "Impact toughness" is one of the main indexes to measure the quality of synthetic industrial diamonds.

Yield strength

Diamond has compressive yield strength of 130–140 GPa. This exceptionally high value, along with the hardness and transparency of diamond, are the reasons that diamond anvil cells are the main tool for high pressure experiments. These anvils have reached pressures of 600 GPa. Much higher pressures may be possible with nanocrystalline diamonds.

Elasticity and tensile strength

Usually, attempting to deform bulk diamond crystal by tension or bending results in brittle fracture. However, when single crystalline diamond is in the form of micro/nanoscale wires or needles (~100–300 nanometers in diameter, micrometers long), they can be elastically stretched by as much as 9–10 percent tensile strain without failure, with a maximum local tensile stress of about 89–98 GPa, very close to the theoretical limit for this material.

Electrical conductivity

Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most diamonds, which are excellent electrical insulators. The conductivity and blue color originate from boron impurity. Boron substitutes for carbon atoms in the diamond lattice, donating a hole into the valence band.

Substantial conductivity is commonly observed in nominally undoped diamond grown by chemical vapor deposition. This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.

Thin needles of diamond can be made to vary their electronic band gap from the normal 5.6 eV to near zero by selective mechanical deformation.

High-purity diamond wafers 5 cm in diameter exhibit perfect resistance in one direction and perfect conductance in the other, creating the possibility of using them for quantum data storage. The material contains only 3 parts per million of nitrogen. The diamond was grown on a stepped substrate, which eliminated cracking.

Surface property

Diamonds are naturally lipophilic and hydrophobic, which means the diamonds' surface cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds. However, when diamond surfaces are chemically modified with certain ions, they are expected to become so hydrophilic that they can stabilize multiple layers of water ice at human body temperature.

The surface of diamonds is partially oxidized. The oxidized surface can be reduced by heat treatment under hydrogen flow. That is to say, this heat treatment partially removes oxygen-containing functional groups. But diamonds (sp3C) are unstable against high temperature (above about 400 °C (752 °F)) under atmospheric pressure. The structure gradually changes into sp2C above this temperature. Thus, diamonds should be reduced below this temperature.

Chemical stability

At room temperature, diamonds do not react with any chemical reagents including strong acids and bases.

In an atmosphere of pure oxygen, diamond has an ignition point that ranges from 690 °C (1,274 °F) to 840 °C (1,540 °F); smaller crystals tend to burn more easily. It increases in temperature from red to white heat and burns with a pale blue flame, and continues to burn after the source of heat is removed. By contrast, in air the combustion will cease as soon as the heat is removed because the oxygen is diluted with nitrogen. A clear, flawless, transparent diamond is completely converted to carbon dioxide; any impurities will be left as ash. Heat generated from cutting a diamond will not ignite the diamond, and neither will a cigarette lighter, but house fires and blow torches are hot enough. Jewelers must be careful when molding the metal in a diamond ring.

Diamond powder of an appropriate grain size (around 50 microns) burns with a shower of sparks after ignition from a flame. Consequently, pyrotechnic compositions based on synthetic diamond powder can be prepared. The resulting sparks are of the usual red-orange color, comparable to charcoal, but show a very linear trajectory which is explained by their high density. Diamond also reacts with fluorine gas above about 700 °C (1,292 °F).

Color

Diamond has a wide band gap of 5.5 eV corresponding to the deep ultraviolet wavelength of 225 nanometers. This means that pure diamond should transmit visible light and appear as a clear colorless crystal. Colors in diamond originate from lattice defects and impurities. The diamond crystal lattice is exceptionally strong, and only atoms of nitrogen, boron, and hydrogen can be introduced into diamond during the growth at significant concentrations (up to atomic percents). Transition metals nickel and cobalt, which are commonly used for growth of synthetic diamond by high-pressure high-temperature techniques, have been detected in diamond as individual atoms; the maximum concentration is 0.01% for nickel and even less for cobalt. Virtually any element can be introduced to diamond by ion implantation.

Nitrogen is by far the most common impurity found in gem diamonds and is responsible for the yellow and brown color in diamonds. Boron is responsible for the blue color. Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds, and plastic deformation of the diamond crystal lattice. Plastic deformation is the cause of color in some brown and perhaps pink and red diamonds. In order of increasing rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red. "Black", or carbonado, diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the crystal lattice, known as a carbon flaw. The most common impurity, nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present. The Gemological Institute of America (GIA) classifies low saturation yellow and brown diamonds as diamonds in the normal color range, and applies a grading scale from "D" (colorless) to "Z" (light yellow). Yellow diamonds of high color saturation or a different color, such as pink or blue, are called fancy colored diamonds and fall under a different grading scale.

In 2008, the Wittelsbach Diamond, a 35.56-carat (7.112 g) blue diamond once belonging to the King of Spain, fetched over US$24 million at a Christie's auction. In May 2009, a 7.03-carat (1.406 g) blue diamond fetched the highest price per carat ever paid for a diamond when it was sold at auction for 10.5 million Swiss francs (6.97 million euros, or US$9.5 million at the time). That record was, however, beaten the same year: a 5-carat (1.0 g) vivid pink diamond was sold for US$10.8 million in Hong Kong on December 1, 2009.

Clarity

Clarity is one of the 4C's (color, clarity, cut and carat weight) that helps in identifying the quality of diamonds. The Gemological Institute of America (GIA) developed 11 clarity scales to decide the quality of a diamond for its sale value. The GIA clarity scale spans from Flawless (FL) to included (I) having internally flawless (IF), very, very slightly included (VVS), very slightly included (VS) and slightly included (SI) in between. Impurities in natural diamonds are due to the presence of natural minerals and oxides. The clarity scale grades the diamond based on the color, size, location of impurity and quantity of clarity visible under 10x magnification. Inclusions in diamond can be extracted by optical methods. The process is to take pre-enhancement images, identifying the inclusion removal part and finally removing the diamond facets and noises.

Fluorescence

Between 25% and 35% of natural diamonds exhibit some degree of fluorescence when examined under invisible long-wave ultraviolet light or higher energy radiation sources such as X-rays and lasers. Incandescent lighting will not cause a diamond to fluoresce. Diamonds can fluoresce in a variety of colors including blue (most common), orange, yellow, white, green and very rarely red and purple. Although the causes are not well understood, variations in the atomic structure, such as the number of nitrogen atoms present are thought to contribute to the phenomenon.

Thermal conductivity

Diamonds can be identified by their high thermal conductivity (900–2320 W•m−1•K−1). Their high refractive index is also indicative, but other materials have similar refractivity.

Modifications and varieties

In addition to cubic crystallizing diamond, the following carbon modifications are also known:

Graphite (hexagonal),

Lonsdaleite (hexagonal),

Chaoite (hexagonal),

Fullerenes (with a few exceptions only synthetic),

Graphene (synthetic).

Diamond is metastable at room temperature and atmospheric pressure. However, the activation energy for the phase transition to the stable modification (graphite) is so high that conversion to graphite practically does not occur at room temperature.

There are several different diamond varieties whose crystal structures exhibit increased lattice defects due to unfavorable growth conditions. These include Ballas (radial, fibrous) and Carbonado (black, porous polycrystalline diamond, which has so far been found exclusively in Central Africa and South America), as well as the pink diamonds from the Argyle Mine. 

Geology

Diamonds are extremely rare, with concentrations of at most parts per billion in source rock. Before the 20th century, most diamonds were found in alluvial deposits. Loose diamonds are also found along existing and ancient shorelines, where they tend to accumulate because of their size and density. Rarely, they have been found in glacial till (notably in Wisconsin and Indiana), but these deposits are not of commercial quality.  These types of deposit were derived from localized igneous intrusions through weathering and transport by wind or water.

Most diamonds come from the Earth's mantle, and most of this section discusses those diamonds. However, there are other sources. Some blocks of the crust, or terranes, have been buried deep enough as the crust thickened so they experienced ultra-high-pressure metamorphism. These have evenly distributed microdiamonds that show no sign of transport by magma. In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for microdiamonds and nanodiamonds to form. Impact-type microdiamonds can be used as an indicator of ancient impact craters. Popigai impact structure in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.

A common misconception is that diamonds form from highly compressed coal. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.

Surface distribution

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford's rule states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores of continents with typical ages of 2.5 billion years or more.  However, there are exceptions. The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is located in a mobile belt, also known as an orogenic belt, a weaker zone surrounding the central craton that has undergone compressional tectonics. Instead of kimberlite, the host rock is lamproite. Lamproites with diamonds that are not economically viable are also found in the United States, India, and Australia. In addition, diamonds in the Wawa belt of the Superior province in Canada and microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre.

Kimberlites can be found in narrow (1 to 4 meters) dikes and sills, and in pipes with diameters that range from about 75 m to 1.5 km. Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles. It is hybrid rock with a chaotic mixture of small minerals and rock fragments (clasts) up to the size of watermelons. They are a mixture of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the eruption. The texture varies with depth. The composition forms a continuum with carbonatites, but the latter have too much oxygen for carbon to exist in a pure form. Instead, it is locked up in the mineral calcite (CaCO

3).

All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals (melilite and kalsilite) that are incompatible with diamond formation. In kimberlite, olivine is large and conspicuous, while lamproite has Ti-phlogopite and lamprophyre has biotite and amphibole. They are all derived from magma types that erupt rapidly from small amounts of melt, are rich in volatiles and magnesium oxide, and are less oxidizing than more common mantle melts such as basalt. These characteristics allow the melts to carry diamonds to the surface before they dissolve.

Exploration

Kimberlite pipes can be difficult to find. They weather quickly (within a few years after exposure) and tend to have lower topographic relief than surrounding rock. If they are visible in outcrops, the diamonds are never visible because they are so rare. In any case, kimberlites are often covered with vegetation, sediments, soils, or lakes. In modern searches, geophysical methods such as aeromagnetic surveys, electrical resistivity, and gravimetry, help identify promising regions to explore. This is aided by isotopic dating and modeling of the geological history. Then surveyors must go to the area and collect samples, looking for kimberlite fragments or indicator minerals. The latter have compositions that reflect the conditions where diamonds form, such as extreme melt depletion or high pressures in eclogites. However, indicator minerals can be misleading; a better approach is geothermobarometry, where the compositions of minerals are analyzed as if they were in equilibrium with mantle minerals.

Finding kimberlites requires persistence, and only a small fraction contain diamonds that are commercially viable. The only major discoveries since about 1980 have been in Canada. Since existing mines have lifetimes of as little as 25 years, there could be a shortage of new natural diamonds in the future.

Ages

Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. Depending on the elemental abundances, one can look at the decay of rubidium to strontium, samarium to neodymium, uranium to lead, argon-40 to argon-39, or rhenium to osmium. Those found in kimberlites have ages ranging from 1 to 3.5 billion years, and there can be multiple ages in the same kimberlite, indicating multiple episodes of diamond formation. The kimberlites themselves are much younger. Most of them have ages between tens of millions and 300 million years old, although there are some older exceptions (Argyle, Premier and Wawa). Thus, the kimberlites formed independently of the diamonds and served only to transport them to the surface. Kimberlites are also much younger than the cratons they have erupted through. The reason for the lack of older kimberlites is unknown, but it suggests there was some change in mantle chemistry or tectonics. No kimberlite has erupted in human history.

Origin in mantle

Most gem-quality diamonds come from depths of 150–250 km in the lithosphere. Such depths occur below cratons in mantle keels, the thickest part of the lithosphere. These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them.

Host rocks in a mantle keel include harzburgite and lherzolite, two type of peridotite. The most dominant rock type in the upper mantle, peridotite is an igneous rock consisting mostly of the minerals olivine and pyroxene; it is low in silica and high in magnesium. However, diamonds in peridotite rarely survive the trip to the surface. Another common source that does keep diamonds intact is eclogite, a metamorphic rock that typically forms from basalt as an oceanic plate plunges into the mantle at a subduction zone.

A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660 km, a region that includes the transition zone. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800 km.

Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition from graphite occurring at greater temperatures as the pressure increases. Thus, underneath continents it becomes stable at temperatures of 950 degrees Celsius and pressures of 4.5 gigapascals, corresponding to depths of 150 kilometers or greater. In subduction zones, which are colder, it becomes stable at temperatures of 800 °C and pressures of 3.5 gigapascals. At depths greater than 240 km, iron–nickel metal phases are present and carbon is likely to be either dissolved in them or in the form of carbides. Thus, the deeper origin of some diamonds may reflect unusual growth environments.

In 2018 the first known natural samples of a phase of ice called Ice VII were found as inclusions in diamond samples. The inclusions formed at depths between 400 and 800 km, straddling the upper and lower mantle, and provide evidence for water-rich fluid at these depths.

Carbon sources

The mantle has roughly one billion gigatonnes of carbon (for comparison, the atmosphere-ocean system has about 44,000 gigatonnes). Carbon has two stable isotopes, 12C and 13C, in a ratio of approximately 99:1 by mass. This ratio has a wide range in meteorites, which implies that it also varied a lot in the early Earth. It can also be altered by surface processes like photosynthesis. The fraction is generally compared to a standard sample using a ratio δ13C expressed in parts per thousand. Common rocks from the mantle such as basalts, carbonatites, and kimberlites have ratios between −8 and −2. On the surface, organic sediments have an average of −25 while carbonates have an average of 0.

Populations of diamonds from different sources have distributions of δ13C that vary markedly. Peridotitic diamonds are mostly within the typical mantle range; eclogitic diamonds have values from −40 to +3, although the peak of the distribution is in the mantle range. This variability implies that they are not formed from carbon that is primordial (having resided in the mantle since the Earth formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds) not necessarily the same tectonic processes that act in the present. Diamond-forming carbon originates in the top 700 kilometers (430 mi) or so of the upper mantle closest to the surface, known as the asthenosphere.

Formation and growth

Diamonds in the mantle form through a metasomatic process where a C–O–H–N–S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C–O–H–N–S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO2 or CO3) or oxidation of a reduced phase such as methane.

Using probes such as polarized light, photoluminescence, and cathodoluminescence, a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.

Transport to the surface

Geological evidence supports a model in which kimberlite magma rises at 4–20 meters per second, creating an upward path by hydraulic fracturing of the rock. As the pressure decreases, a vapor phase exsolves from the magma, and this helps to keep the magma fluid. At the surface, the initial eruption explodes out through fissures at high speeds (over 200 m/s (450 mph)). Then, at lower pressures, the rock is eroded, forming a pipe and producing fragmented rock (breccia). As the eruption wanes, there is pyroclastic phase and then metamorphism and hydration produces serpentinites.

Double diamonds

In rare cases, diamonds have been found that contain a cavity within which is a second diamond. The first double diamond, the Matryoshka, was found by Alrosa in Yakutia, Russia, in 2019. Another one was found in the Ellendale Diamond Field in Western Australia in 2021.

In space

Although diamonds on Earth are rare, they are very common in space. In meteorites, about three percent of the carbon is in the form of nanodiamonds, having diameters of a few nanometers. Sufficiently small diamonds can form in the cold of space because their lower surface energy makes them more stable than graphite. The isotopic signatures of some nanodiamonds indicate they were formed outside the Solar System in stars.

High pressure experiments predict that large quantities of diamonds condense from methane into a "diamond rain" on the ice giant planets Uranus and Neptune. Some extrasolar planets may be almost entirely composed of diamond.

Diamonds may exist in carbon-rich stars, particularly white dwarfs. One theory for the origin of carbonado, the toughest form of diamond, is that it originated in a white dwarf or supernova. Diamonds formed in stars may have been the first minerals.

Industry

The most familiar uses of diamonds today are as gemstones used for adornment, and as industrial abrasives for cutting hard materials. The markets for gem-grade and industrial-grade diamonds value diamonds differently.

Gem-grade diamonds

The dispersion of white light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology developed methods of grading diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are its mass in carats (a carat being equal to 0.2 grams), cut (quality of the cut is graded according to proportions, symmetry and polish), color (how close to white or colorless; for fancy diamonds how intense is its hue), and clarity (how free is it from inclusions). A large, flawless diamond is known as a paragon.

A large trade in gem-grade diamonds exists. Although most gem-grade diamonds are sold newly polished, there is a well-established market for resale of polished diamonds (e.g. pawnbroking, auctions, second-hand jewelry stores, diamantaires, bourses, etc.). One hallmark of the trade in gem-quality diamonds is its remarkable concentration: wholesale trade and diamond cutting is limited to just a few locations; in 2003, 92% of the world's diamonds were cut and polished in Surat, India. Other important centers of diamond cutting and trading are the Antwerp diamond district in Belgium, where the International Gemological Institute is based, London, the Diamond District in New York City, the Diamond Exchange District in Tel Aviv and Amsterdam. One contributory factor is the geological nature of diamond deposits: several large primary kimberlite-pipe mines each account for significant portions of market share (such as the Jwaneng mine in Botswana, which is a single large-pit mine that can produce between 12,500,000 and 15,000,000 carats (2,500 and 3,000 kg) of diamonds per year). Secondary alluvial diamond deposits, on the other hand, tend to be fragmented amongst many different operators because they can be dispersed over many hundreds of square kilometers (e.g., alluvial deposits in Brazil).

The production and distribution of diamonds is largely consolidated in the hands of a few key players, and concentrated in traditional diamond trading centers, the most important being Antwerp, where 80% of all rough diamonds, 50% of all cut diamonds and more than 50% of all rough, cut and industrial diamonds combined are handled. This makes Antwerp a de facto "world diamond capital". The city of Antwerp also hosts the Antwerpsche Diamantkring, created in 1929 to become the first and biggest diamond bourse dedicated to rough diamonds. Another important diamond center is New York City, where almost 80% of the world's diamonds are sold, including auction sales.

The De Beers company, as the world's largest diamond mining company, holds a dominant position in the industry, and has done so since soon after its founding in 1888 by the British businessman Cecil Rhodes. De Beers is currently the world's largest operator of diamond production facilities (mines) and distribution channels for gem-quality diamonds. The Diamond Trading Company (DTC) is a subsidiary of De Beers and markets rough diamonds from De Beers-operated mines. De Beers and its subsidiaries own mines that produce some 40% of annual world diamond production. For most of the 20th century over 80% of the world's rough diamonds passed through De Beers, but by 2001–2009 the figure had decreased to around 45%, and by 2013 the company's market share had further decreased to around 38% in value terms and even less by volume. De Beers sold off the vast majority of its diamond stockpile in the late 1990s – early 2000s and the remainder largely represents working stock (diamonds that are being sorted before sale). This was well documented in the press but remains little known to the general public.

As a part of reducing its influence, De Beers withdrew from purchasing diamonds on the open market in 1999 and ceased, at the end of 2008, purchasing Russian diamonds mined by the largest Russian diamond company Alrosa. As of January 2011, De Beers states that it only sells diamonds from the following four countries: Botswana, Namibia, South Africa and Canada. Alrosa had to suspend their sales in October 2008 due to the global energy crisis, but the company reported that it had resumed selling rough diamonds on the open market by October 2009. Apart from Alrosa, other important diamond mining companies include BHP, which is the world's largest mining company; Rio Tinto, the owner of the Argyle (100%), Diavik (60%), and Murowa (78%) diamond mines; and Petra Diamonds, the owner of several major diamond mines in Africa.

Further down the supply chain, members of The World Federation of Diamond Bourses (WFDB) act as a medium for wholesale diamond exchange, trading both polished and rough diamonds. The WFDB consists of independent diamond bourses in major cutting centers such as Tel Aviv, Antwerp, Johannesburg and other cities across the US, Europe and Asia. In 2000, the WFDB and The International Diamond Manufacturers Association established the World Diamond Council to prevent the trading of diamonds used to fund war and inhumane acts. WFDB's additional activities include sponsoring the World Diamond Congress every two years, as well as the establishment of the International Diamond Council (IDC) to oversee diamond grading.

Once purchased by Sightholders (which is a trademark term referring to the companies that have a three-year supply contract with DTC), diamonds are cut and polished in preparation for sale as gemstones ('industrial' stones are regarded as a by-product of the gemstone market; they are used for abrasives). The cutting and polishing of rough diamonds is a specialized skill that is concentrated in a limited number of locations worldwide. Traditional diamond cutting centers are Antwerp, Amsterdam, Johannesburg, New York City, and Tel Aviv. Recently, diamond cutting centers have been established in China, India, Thailand, Namibia and Botswana. Cutting centers with lower cost of labor, notably Surat in Gujarat, India, handle a larger number of smaller carat diamonds, while smaller quantities of larger or more valuable diamonds are more likely to be handled in Europe or North America. The recent expansion of this industry in India, employing low cost labor, has allowed smaller diamonds to be prepared as gems in greater quantities than was previously economically feasible.

Diamonds prepared as gemstones are sold on diamond exchanges called bourses. There are 28 registered diamond bourses in the world. Bourses are the final tightly controlled step in the diamond supply chain; wholesalers and even retailers are able to buy relatively small lots of diamonds at the bourses, after which they are prepared for final sale to the consumer. Diamonds can be sold already set in jewelry, or sold unset ("loose"). According to the Rio Tinto, in 2002 the diamonds produced and released to the market were valued at US$9 billion as rough diamonds, US$14 billion after being cut and polished, US$28 billion in wholesale diamond jewelry, and US$57 billion in retail sales.

Cutting

Mined rough diamonds are converted into gems through a multi-step process called "cutting". Diamonds are extremely hard, but also brittle and can be split up by a single blow. Therefore, diamond cutting is traditionally considered as a delicate procedure requiring skills, scientific knowledge, tools and experience. Its final goal is to produce a faceted jewel where the specific angles between the facets would optimize the diamond luster, that is dispersion of white light, whereas the number and area of facets would determine the weight of the final product. The weight reduction upon cutting is significant and can be of the order of 50%. Several possible shapes are considered, but the final decision is often determined not only by scientific, but also practical considerations. For example, the diamond might be intended for display or for wear, in a ring or a necklace, singled or surrounded by other gems of certain color and shape. Some of them may be considered as classical, such as round, pear, marquise, oval, hearts and arrows diamonds, etc. Some of them are special, produced by certain companies, for example, Phoenix, Cushion, Sole Mio diamonds, etc.

The most time-consuming part of the cutting is the preliminary analysis of the rough stone. It needs to address a large number of issues, bears much responsibility, and therefore can last years in case of unique diamonds. The following issues are considered:

The hardness of diamond and its ability to cleave strongly depend on the crystal orientation. Therefore, the crystallographic structure of the diamond to be cut is analyzed using X-ray diffraction to choose the optimal cutting directions.

Most diamonds contain visible non-diamond inclusions and crystal flaws. The cutter has to decide which flaws are to be removed by the cutting and which could be kept.

Splitting a diamond with a hammer is difficult, a well-calculated, angled blow can cut the diamond, piece-by-piece, but it can also ruin the diamond itself. Alternatively, it can be cut with a diamond saw, which is a more reliable method.

After initial cutting, the diamond is shaped in numerous stages of polishing. Unlike cutting, which is a responsible but quick operation, polishing removes material by gradual erosion and is extremely time-consuming. The associated technique is well developed; it is considered as a routine and can be performed by technicians. After polishing, the diamond is reexamined for possible flaws, either remaining or induced by the process. Those flaws are concealed through various diamond enhancement techniques, such as repolishing, crack filling, or clever arrangement of the stone in the jewelry. Remaining non-diamond inclusions are removed through laser drilling and filling of the voids produced.

Fancy Diamonds

The name Fancy Diamonds, also called Fancys for short, refers to colored diamonds. While most diamonds are colored, many are unattractive. The diamond's natural color can be dominated by all shades of gray, yellow, green, and brown; occasionally it even changes within a single stone. Pure, intense colors are rare and valuable; accordingly, better prices are paid for them, which can sometimes be considerably higher than the standard for colorless diamonds. Pink diamonds, for example, are valued 50 times more highly than white ones. Statistically, there is on average only one "Fancy" diamond in every 100,000 diamonds. Yellow and brown tones, which make up more than 80 percent of all colored diamonds, are not fancy in the strict sense. Canary yellow or cognac golden brown, on the other hand, are fancy colors. The Aurora Collection is a large collection of colored diamonds.

A diamond can change its color due to radioactive irradiation. Artificial irradiation is often followed by a heat treatment, which also affects the color. For artificially irradiated diamonds, the color treatment must be stated on the certificate, as they are of significantly lower value.

The color names are chosen for marketing purposes: Golden Orange, Lemon, Chocolate, Noir/Black, Electric Blue. The first major source of fancy diamonds was discovered in South Africa in 1867. Since the 1980s, the Argyle Mine in Australia has been the most important source of pink to red fancy diamonds.

There are seven fancy colors, along with many other intermediate colors such as gold, gray, and yellow-green. Each color is produced by a different substance:

Canary yellow: Nitrogen is responsible for the yellow tones. The higher the nitrogen content, the more intense the yellow or green tone. The most famous and probably largest yellow diamond is the Tiffany Diamond, weighing 128.51 carats and weighing 287.42 carats. Yellow is the most common color of diamonds after white and, along with brown, is another famous yellow diamond. The Golden Jubilee.

Brown: Defects in the crystal lattice are responsible for the brown tones. The largest brown cut diamond is the Earth Star at 111.6 carats. The largest brown diamond ever found is probably the Lesotho Brown at 601 carats.

Blue: The element boron is responsible for the blue color of diamonds. The largest and most famous blue diamond is the supposedly cursed Hope Diamond, which weighed 112.5 carats uncut and now weighs 45.52 carats in its cut state. In 2014, a pear-shaped blue diamond from the Rachel Lambert Mellon collection, weighing "only" 9.75 carats (1.95 grams), was auctioned by Sotheby's in New York for $32.6 million (€26.3 million), making it the highest carat value ever achieved for a diamond, at $3.35 million. The Oppenheimer Blue, weighing 14.62 carats and classified by the Gemological Institute of America as the largest blue diamond, was auctioned in May 2016 for a record sum of $57.5 million (€51.3 million). 

Green: The most famous and perhaps largest diamond of this color is the Dresden Green Diamond, weighing 41.0 carats (119.5 carats uncut) (on display in the Green Vault). Green diamonds are very rare. The green color can be caused by radiation defects.

Red: Crystal defects are likely responsible for this color. The largest red diamond ever found is the Australian Red Diamond, with a rough weight of 35 carats. The largest cut red diamond is the Australian Red Shield, at 5.11 carats. Pure red diamonds are the rarest of all diamonds. 90 percent of red diamonds come from the Argyle Mine in Australia. Only ten purple diamonds exist, the largest of which weighs 3 carats. All of them also came from the Argyle Mine. Red diamonds are the most expensive of all diamonds.

Pink or rose: Pink diamonds are often counted among the red diamonds. Here, too, crystal impurities are responsible for the color. The largest rough diamond of this color is the Darya-i-Nur (also Daryā-ye nūr) with a weight depending on the source between 182 and 186 carats and a size of 41.40 mm × 29.50 mm × 12.15 mm, the largest cut diamond the Steinmetz Pink, now Pink Star with 59.6 carats, which was auctioned on 13 November 2013 in Geneva and achieved the highest price to date (52 million pounds sterling) for a stone. The stone was renamed The Pink Dream after the purchase. In February 2014, Sotheby's in Geneva announced that the buyer, gem cutter Isaac Wolf, could not raise the purchase price. Due to the agreement with the consignor, the auction house had to take possession of the stone for approximately 72 million US dollars. Of the 66 largest diamonds, only one is pink.

Orange A rare 14.82-carat orange diamond owned by the Gemological Institute of America fetched a record $35.5 million at Christie's auction in Geneva on November 12, 2013. As with yellow diamonds, nitrogen is responsible for the orange color.

Black diamonds

Black diamonds became popular as fashion jewelry in the 1990s. Besides the rare, naturally occurring Carbonado, which probably came to Earth via meteorites, there are black diamonds that have formed solely from the Earth. The most famous is the 67.5-carat Black Orlov. The largest black diamond found to date is The Enigma, in its cut state with exactly 555.55 carats and 55 facets, which was auctioned at Sotheby's in February 2022 for 3.75 million euros. Black diamonds are often created from (low-quality) bright specimens by intensive neutron irradiation and offered as jewelry stones.

Trade

A large proportion of uncut and cut diamonds are traded through diamond bourses, of which there are 30 worldwide. One of the most important is based in Antwerp. The World Federation of Diamond Bourses is also based there.

De Beers is the most important producer and trader and held a monopoly position for a long time. The company was controversial, especially because of its practice of buying up surplus diamonds and thus keeping diamond prices stable. 

Industrial diamonds constitute by far the largest share of traded diamonds – only 3% of industrial diamonds are of natural origin. This 3% represents the 70% of mined natural diamonds that do not meet the requirements of jewelry production. 

Marketing

Marketing has significantly affected the image of diamond as a valuable commodity.

N. W. Ayer & Son, the advertising firm retained by De Beers in the mid-20th century, succeeded in reviving the American diamond market and the firm created new markets in countries where no diamond tradition had existed before. N. W. Ayer's marketing included product placement, advertising focused on the diamond product itself rather than the De Beers brand, and associations with celebrities and royalty. Without advertising the De Beers brand, De Beers was advertising its competitors' diamond products as well, but this was not a concern as De Beers dominated the diamond market throughout the 20th century. De Beers' market share dipped temporarily to second place in the global market below Alrosa in the aftermath of the global economic crisis of 2008, down to less than 29% in terms of carats mined, rather than sold. The campaign lasted for decades but was effectively discontinued by early 2011. De Beers still advertises diamonds, but the advertising now mostly promotes its own brands, or licensed product lines, rather than completely "generic" diamond products. The campaign was perhaps best captured by the slogan "a diamond is forever". This slogan is now being used by De Beers Diamond Jewelers, a jewelry firm which is a 50/50% joint venture between the De Beers mining company and LVMH, the luxury goods conglomerate.

Brown-colored diamonds constituted a significant part of the diamond production, and were predominantly used for industrial purposes. They were seen as worthless for jewelry (not even being assessed on the diamond color scale). After the development of Argyle diamond mine in Australia in 1986, and marketing, brown diamonds have become acceptable gems. The change was mostly due to the numbers: the Argyle mine, with its 35,000,000 carats (7,000 kg) of diamonds per year, makes about one-third of global production of natural diamonds; 80% of Argyle diamonds are brown.

Industrial-grade diamonds

Industrial diamonds are valued mostly for their hardness and thermal conductivity, making many of the gemological characteristics of diamonds, such as the 4 Cs, irrelevant for most applications. Eighty percent of mined diamonds (equal to about 135,000,000 carats (27,000 kg) annually) are unsuitable for use as gemstones and are used industrially. In addition to mined diamonds, synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; in 2014, 4,500,000,000 carats (900,000 kg) of synthetic diamonds were produced, 90% of which were produced in China. Approximately 90% of diamond grinding grit is currently of synthetic origin.

The boundary between gem-quality diamonds and industrial diamonds is poorly defined and partly depends on market conditions (for example, if demand for polished diamonds is high, some lower-grade stones will be polished into low-quality or small gemstones rather than being sold for industrial use). Within the category of industrial diamonds, there is a sub-category comprising the lowest-quality, mostly opaque stones, which are known as bort.

Industrial use of diamonds has historically been associated with their hardness, which makes diamond the ideal material for cutting and grinding tools. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial applications of this property include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds (bort) with more flaws and poorer color than gems, are used for such purposes. Diamond is not suitable for machining ferrous alloys at high speeds, as carbon is soluble in iron at the high temperatures created by high-speed machining, leading to greatly increased wear on diamond tools compared to alternatives.

Specialized applications include use in laboratories as containment for high-pressure experiments, high-performance bearings, and limited use in specialized windows. With the continuing advances being made in the production of synthetic diamonds, future applications are becoming feasible. The high thermal conductivity of diamond makes it suitable as a heat sink for integrated circuits in electronics.

Mining

Approximately 130,000,000 carats (26,000 kg) of diamonds are mined annually, with a total value of nearly US$9 billion, and about 100,000 kg (220,000 lb) are synthesized annually.

Roughly 49% of diamonds originate from Central and Southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia. They are mined from kimberlite and lamproite volcanic pipes, which can bring diamond crystals, originating from deep within the Earth where high pressures and temperatures enable them to form, to the surface. The mining and distribution of natural diamonds are subjects of frequent controversy such as concerns over the sale of blood diamonds or conflict diamonds by African paramilitary groups. The diamond supply chain is controlled by a limited number of powerful businesses, and is also highly concentrated in a small number of locations around the world.

Only a very small fraction of the diamond ore consists of actual diamonds. The ore is crushed, during which care is required not to destroy larger diamonds, and then sorted by density. Today, diamonds are located in the diamond-rich density fraction with the help of X-ray fluorescence, after which the final sorting steps are done by hand. Before the use of X-rays became commonplace, the separation was done with grease belts; diamonds have a stronger tendency to stick to grease than the other minerals in the ore.

Historically, diamonds were found only in alluvial deposits in Guntur and Krishna district of the Krishna River delta in Southern India. India led the world in diamond production from the time of their discovery in approximately the 9th century BC to the mid-18th century AD, but the commercial potential of these sources had been exhausted by the late 18th century and at that time India was eclipsed by Brazil where the first non-Indian diamonds were found in 1725. Currently, one of the most prominent Indian mines is located at Panna.

Diamond extraction from primary deposits (kimberlites and lamproites) started in the 1870s after the discovery of the Diamond Fields in South Africa. Production has increased over time and now an accumulated total of 4,500,000,000 carats (900,000 kg) have been mined since that date. Twenty percent of that amount has been mined in the last five years, and during the last 10 years, nine new mines have started production; four more are waiting to be opened soon. Most of these mines are located in Canada, Zimbabwe, Angola, and one in Russia.

In the U.S., diamonds have been found in Arkansas, Colorado, New Mexico, Wyoming, and Montana. In 2004, the discovery of a microscopic diamond in the U.S. led to the January 2008 bulk-sampling of kimberlite pipes in a remote part of Montana. The Crater of Diamonds State Park in Arkansas is open to the public, and is the only mine in the world where members of the public can dig for diamonds.

Today, most commercially viable diamond deposits are in Russia (mostly in Sakha Republic, for example Mir pipe and Udachnaya pipe), Botswana, Australia (Northern and Western Australia) and the Democratic Republic of the Congo. In 2005, Russia produced almost one-fifth of the global diamond output, according to the British Geological Survey. Australia boasts the richest diamantiferous pipe, with production from the Argyle diamond mine reaching peak levels of 42 metric tons per year in the 1990s. There are also commercial deposits being actively mined in the Northwest Territories of Canada and Brazil. Diamond prospectors continue to search the globe for diamond-bearing kimberlite and lamproite pipes.

Technical uses

Historical use

Diamonds not intended for use as gemstones, artificially produced industrial diamonds, and diamond dust made from both are called “bort” (obsolete “bord”).

Knowledge of the great hardness of diamonds is ancient. Accordingly, the mineral was already used in the pre-industrial age not only for jewelry purposes, but also for all kinds of craft processes. Glaziers used small pieces, cast in tin solder and provided with a handle, to cut glass. Small diamond chips were also used for engraving and drilling hard materials and in lithography. Otherwise unusable chips, very small or impure crystals, were pulverized in hollow cast-iron cylinders or flat mortars to form abrasive powder ("diamond rind"). To prevent chips from flying off, the substance was moistened with a little oil. 

“The small, impure, and poorly colored crystals and grains of this substance are pulverized and are then known as diamond boride.”

– Johann Reinhard Blum: J. Reinhard Blum: Lithurgy or Minerals and Rock Types Systematically Treated According to Their Application in Economic, Artistic, and Technical Aspects. E. Schweizerbart's Publishing House, Stuttgart 1840, p. 104.

Diamond drills for deep drilling were introduced in the industrial age in 1864, and their use quickly spread. The experience gained from this led to saw blades in stone processing also being studded with diamonds. 

Until the 20th century, natural diamonds were and are still used in the machining of materials. Grinding and emery stones were widely used, as were tools for turning chilled cast iron, steel, and hard rubber rollers. A special use, until the production of artificial diamonds, was the production of drill bits for working with and in rock. 

Residues from the further processing of diamonds or fractions unsuitable for gemstones are used worldwide for technical purposes. Larger deposits of industrial diamonds include deposits in the Democratic Republic of Congo. 

Tools

Due to its great hardness, wear resistance and thermal conductivity, diamond is used in industrial production primarily as a cutting material, i.e. for drills, milling tools, drill bits and turning tools, and also as an abrasive for grinding wheels or as an additive in polishing pastes. As a cutting material, diamond can be used in the form of monocrystalline diamond, which consists of a single piece. More common are tools made from polycrystalline diamond, in which small diamond grains have been sintered with a binding agent to form a larger tool. The binding agent serves to fill the gaps between the grains. In diamond abrasives, only granular materials are used. In some areas it is extremely economical to use diamond tools, which can minimize downtime and tool changeover times. The required surface quality can often be achieved in a single step using diamond tools without additional machining. They are frequently used for the precision machining of aluminium and copper.

Diamond tools are not suitable for machining steel because they convert into graphite at the high temperatures encountered there and the carbon atoms diffuse into the steel.

Diamond tips for glass cutters and impeders for hardness testers are also well known.

Diamond-like layers

Thin CVD layers of diamond-like carbon serve as wear protection.

By adding boron, phosphorus, or nitrogen, diamond can be made conductive and act as a semiconductor or even a superconductor. Use in electronic circuits could lead to higher switching speeds due to the high mobility of charge carriers in diamond single crystals and the good temperature tolerance. Electrically conductive diamond coatings can be used to produce electrodes for use in chemical reactions that must withstand highly reactive radicals. On a large scale, wastewater treatment and purification are coming into focus, where CVD diamond electrodes are used for the oxidation and disinfection of, for example, wastewater and process water.

The coating of silicon wafers with artificial diamond has already been realized and can be used by the semiconductor industry to achieve better cooling of electronic circuits.

Optics

One application area for pure diamonds is infrared spectroscopy and the production of lenses and windows.

Miscellaneous

The styluses of higher-quality cartridges for playing records are made of diamond. These diamonds have a special shape and are seated in a needle cantilever made of aluminum or boron.

A multitude of tiny diamonds in a free-flowing form were used in an hourglass. 

Diamond anvil cells are used in materials research to achieve very high pressures in the gigapascal range.

Synthetics, simulants, and enhancements

Synthetics

Synthetic diamonds are diamonds manufactured in a laboratory, as opposed to diamonds mined from the Earth. The gemological and industrial uses of diamond have created a large demand for rough stones. This demand has been satisfied in large part by synthetic diamonds, which have been manufactured by various processes for more than half a century. However, in recent years it has become possible to produce gem-quality synthetic diamonds of significant size. It is possible to make colorless synthetic gemstones that, on a molecular level, are identical to natural stones and so visually similar that only a gemologist with special equipment can tell the difference.

The majority of commercially available synthetic diamonds are yellow and are produced by so-called high-pressure high-temperature (HPHT) processes. The yellow color is caused by nitrogen impurities. Other colors may also be reproduced such as blue, green or pink, which are a result of the addition of boron or from irradiation after synthesis.

Another popular method of growing synthetic diamond is chemical vapor deposition (CVD). The growth occurs under low pressure (below atmospheric pressure). It involves feeding a mixture of gases (typically 1 to 99 methane to hydrogen) into a chamber and splitting them into chemically active radicals in a plasma ignited by microwaves, hot filament, arc discharge, welding torch, or laser. This method is mostly used for coatings, but can also produce single crystals several millimeters in size (see picture).

As of 2010, nearly all 5,000 million carats (1,000 tonnes) of synthetic diamonds produced per year are for industrial use. Around 50% of the 133 million carats of natural diamonds mined per year end up in industrial use. Mining companies' expenses average 40 to 60 US dollars per carat for natural colorless diamonds, while synthetic manufacturers' expenses average $2,500 per carat for synthetic, gem-quality colorless diamonds.: 79  However, a purchaser is more likely to encounter a synthetic when looking for a fancy-colored diamond because only 0.01% of natural diamonds are fancy-colored, while most synthetic diamonds are colored in some way.

Manufacturing process

High-pressure, high-temperature process

Since 1955, it has been possible to produce artificial diamonds using the high- pressure high-temperature (HPHT) process. In this process, graphite is compressed in a hydraulic press at pressures of up to 6 gigapascals (60,000 bar) and temperatures of over 1500°C. Under these conditions, diamond is the thermodynamically more stable form of carbon, so the graphite is converted to diamond. This conversion process can be accelerated by adding a catalyst (usually iron carbonyl). Even with a catalyst, the conversion process still takes several weeks. Analogous to diamond, cubic boron nitride (CBN) can be produced from the hexagonal modification of boron nitride using the high-pressure high-temperature synthesis. CBN does not quite reach the hardness of diamond up to temperatures of around 700°C, but is resistant to oxygen, for example, at high temperatures.

This process is also used in diamond burials, where carbon from the ashes of the deceased is pressed into diamonds.

Detonation synthesis

Other processes for generating high temperatures and pressures are so-called detonation synthesis and shock wave synthesis. In detonation synthesis, a distinction is made between the detonation of a mixture of graphite and explosive material and the detonation of explosive materials alone. In the latter case, an explosive mixture of TNT (trinitrotoluene) and RDX (hexogen) is ignited in a sealed container. The explosive supplies the required energy and is also a carbon carrier. In shock wave synthesis, the pressure required to convert carbon material into diamond is brought about by the effect of an external shock wave, also triggered by an explosion. The explosion compresses a capsule filled with carbon material. This force causes the internal carbon material to convert into diamond. Industrial diamond is just as hard as natural diamond.

Layers

An alternative to producing artificial diamonds is coating substrates using chemical vapor deposition (CVD). This involves depositing a CVD diamond layer a few micrometers thick onto the substrates, such as hard metal tools, in a vacuum chamber. The starting material is typically a gas mixture of methane and hydrogen, with the former serving as the carbon source.

According to Ostwald's rule of stages, metastable diamond should be the primary deposit; according to the Ostwald-Volmer rule, graphite is the primary deposit due to its lower density. Atomic hydrogen can be used to selectively decompose graphite, promoting the formation of diamond. Atomic hydrogen (H) is formed in a thermally or electrically heated plasma from molecular hydrogen gas (H2). The substrate temperature must be below 1000 °C to prevent the conversion to stable graphite. Growth rates of several micrometers per hour can then be achieved.

As a further development, plasma coating techniques such as PECVD can be used to produce layers of so-called diamond -like carbon (DLC) that are just a few nanometers to micrometers thick. These layers combine very high hardness with very good sliding friction properties. Depending on the coating parameters, they contain a mixture of sp 2 and sp 3 hybridized carbon atoms. These layers are therefore not diamond. However, they do have certain properties of diamond and are therefore referred to as "diamond-like" or "diamond-like". By controlling the process and choosing the precursor material, many types of carbon layers can be produced, from hard, hydrogen-free to very elastic, hydrogen-containing.

Homo- and heteroepitaxy

Microwave plasma-assisted chemical vapor deposition (MWPCVD) makes it possible to produce thick diamond bodies on thin diamond substrates or on lattice-matched foreign substrates (heteroepitaxial). The latter process was used in 2016 to successfully produce a disk-shaped diamond weighing 155 carats and measuring 92 mm in diameter.  The process involves, on the one hand, carbon being released from hydrocarbons (e.g., methane) in the plasma and deposited there, and, on the other hand, a high proportion of atomic hydrogen in the plasma ensures that all non-diamond-like deposited structures are removed. The most promising substrate for the heteroepitaxial production of diamond disks in 2008 is a multilayer structure consisting of an iridium layer on yttrium -stabilized zirconium(IV) oxide (YSZ), which was deposited on a single- crystal silicon wafer. 

Further processing

This commercially successful method produces diamond powder in various finenesses. The synthetically produced rough diamonds are first mechanically crushed (grinding in ball mills). Impurities from residues of the reactants on the surface of the diamond particles, such as non-combustible impurities or unconverted graphite residues, are chemically removed. Coarser grain sizes are classified by sieving. Micro-grains, however, must be sedimented. For this purpose, the diamond powder is placed in a basin of water. Using Stokes' law, the sedimentation rate of a spherical particle can be calculated. The upper layers of the water-diamond powder mixture are carefully vacuumed off after a certain sedimentation period and physically dried.

Magnetic diamond

At Rensselaer Polytechnic Institute in Troy, New York, researchers succeeded in producing magnetic diamonds. They are only five nanometers in size and possess their own magnetic field. This effect is based on a defect in the crystal lattice. Applications for this health-compatible carbon are predicted, primarily in medicine. 

Monocrystalline diamond powder

Monocrystalline industrial diamond (single crystal) is relatively inexpensive and can be produced in large quantities. It is therefore widely used in industrial grinding, lapping, and polishing processes. The diamond has a monocrystalline lattice structure, with slip planes oriented parallel to the optical axis (111 plane). When stressed, the monocrystalline diamond grain breaks along the parallel cleavage planes. This creates blocky grains with sharp cutting edges. Figuratively speaking, a monocrystalline diamond grain breaks like a salami being sliced ("salami slice model").

Polycrystalline diamond powder

A polycrystalline (industrial) diamond is composed of numerous tiny diamond grains. Under stress, small corners and edges break away from the diamond grain, continually creating new, sharp cutting edges (self-sharpening effect). This characteristic allows for high stock removal rates and the finest surface finishes. It is suitable for lapping and polishing extremely hard materials, such as ceramics or sapphire glass.

Nanodiamond

Nanodiamond powder is used in various applications and research areas. The high volume-to-surface ratio creates new physical and chemical properties. For example, nanodiamonds have perfect lubricating properties and are therefore added to lubricating oils. Another potential application for nanodiamonds is cancer therapy. 

Simulants

A diamond simulant is a non-diamond material that is used to simulate the appearance of a diamond, and may be referred to as diamante. Cubic zirconia is the most common. The gemstone moissanite (silicon carbide) can be treated as a diamond simulant, though more costly to produce than cubic zirconia. Both are produced synthetically.

Enhancements

Diamond enhancements are specific treatments performed on natural or synthetic diamonds (usually those already cut and polished into a gem), which are designed to better the gemological characteristics of the stone in one or more ways. These include laser drilling to remove inclusions, application of sealants to fill cracks, treatments to improve a white diamond's color grade, and treatments to give fancy color to a white diamond.

Coatings are increasingly used to give a diamond simulant such as cubic zirconia a more "diamond-like" appearance. One such substance is diamond-like carbon—an amorphous carbonaceous material that has some physical properties similar to those of the diamond. Advertising suggests that such a coating would transfer some of these diamond-like properties to the coated stone, hence enhancing the diamond simulant. Techniques such as Raman spectroscopy should easily identify such a treatment.

Identification

Early diamond identification tests included a scratch test relying on the superior hardness of diamond. This test is destructive, as a diamond can scratch another diamond, and is rarely used nowadays. Instead, diamond identification relies on its superior thermal conductivity. Electronic thermal probes are widely used in the gemological centers to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about two to three seconds.

Whereas the thermal probe can separate diamonds from most of their simulants, distinguishing between various types of diamond, for example synthetic or natural, irradiated or non-irradiated, etc., requires more advanced, optical techniques. Those techniques are also used for some diamonds simulants, such as silicon carbide, which pass the thermal conductivity test. Optical techniques can distinguish between natural diamonds and synthetic diamonds. They can also identify the vast majority of treated natural diamonds. "Perfect" crystals (at the atomic lattice level) have never been found, so both natural and synthetic diamonds always possess characteristic imperfections, arising from the circumstances of their crystal growth, that allow them to be distinguished from each other.

Laboratories use techniques such as spectroscopy, microscopy, and luminescence under shortwave ultraviolet light to determine a diamond's origin. They also use specially made instruments to aid them in the identification process. Two screening instruments are the DiamondSure and the DiamondView, both produced by the DTC and marketed by the GIA.

Several methods for identifying synthetic diamonds can be performed, depending on the method of production and the color of the diamond. CVD diamonds can usually be identified by an orange fluorescence. D–J colored diamonds can be screened through the Swiss Gemmological Institute's Diamond Spotter. Stones in the D–Z color range can be examined through the DiamondSure UV/visible spectrometer, a tool developed by De Beers. Similarly, natural diamonds usually have minor imperfections and flaws, such as inclusions of foreign material, that are not seen in synthetic diamonds.

Screening devices based on diamond type detection can be used to make a distinction between diamonds that are certainly natural and diamonds that are potentially synthetic. Those potentially synthetic diamonds require more investigation in a specialized lab. Examples of commercial screening devices are D-Screen (WTOCD / HRD Antwerp), Alpha Diamond Analyzer (Bruker / HRD Antwerp), and D-Secure (DRC Techno).

Political issues

In some of the more politically unstable central African and west African countries, revolutionary groups have taken control of diamond mines, using proceeds from diamond sales to finance their operations. Diamonds sold through this process are known as conflict diamonds or blood diamonds.

In response to public concerns that their diamond purchases were contributing to war and human rights abuses in central and western Africa, the United Nations, the diamond industry and diamond-trading nations introduced the Kimberley Process in 2002. The Kimberley Process aims to ensure that conflict diamonds do not become intermixed with the diamonds not controlled by such rebel groups. This is done by requiring diamond-producing countries to provide proof that the money they make from selling the diamonds is not used to fund criminal or revolutionary activities. Although the Kimberley Process has been moderately successful in limiting the number of conflict diamonds entering the market, some still find their way in. According to the International Diamond Manufacturers Association, conflict diamonds constitute 2–3% of all diamonds traded. Two major flaws still hinder the effectiveness of the Kimberley Process: (1) the relative ease of smuggling diamonds across African borders, and (2) the violent nature of diamond mining in nations that are not in a technical state of war and whose diamonds are therefore considered "clean".

The Canadian Government has set up a body known as the Canadian Diamond Code of Conduct to help authenticate Canadian diamonds. This is a stringent tracking system of diamonds and helps protect the "conflict free" label of Canadian diamonds.

Mineral resource exploitation in general causes irreversible environmental damage, which must be weighed against the socio-economic benefits to a country.

Social influences

While the majority of today's diamonds are mined using modern means by a very few internationally operating corporations like De Beers, the exorbitant price paid for diamonds results in excavations taking place under deplorable and sometimes life-threatening conditions, especially in the world's underdeveloped regions and crisis zones. Even when individual miners find diamonds, the rough diamonds are usually sold cheaply to local rulers, leaving only a fraction of the profits for the actual miners.

Profits from the diamond trade are also used to finance several civil wars on the African continent, such as in the Democratic Republic of Congo. For this reason, efforts are being made to stop the trade in these blood diamonds, or conflict diamonds. However, it is not easy to determine a diamond's origin by looking at it, and certificates supposed to provide proof of origin are often forged. It is possible to individually mark diamonds with lasers. The origin can then be verified using this identification number.

In the illegal arms trade, especially in West Africa, payment in diamonds is not uncommon. The reasons for this are obvious: they are small (therefore easy to transport and conceal), valuable, and their value fluctuates little. None of this is usually the case with local currencies.

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