Diamond one of the forms of carbon
Diamond is one of the two best known forms (or allotropes) of carbon, whose hardness and high dispersion of light make it useful for industrial applications and jewelry (the other equally well known allotrope is graphite). Diamonds are specifically renowned as a mineral with superlative physical qualities - they make excellent abrasives because they can only be scratched by other diamonds, which also means they hold a polish extremely well and retain luster. About 130 million carats (26,000 kg) are mined annually, with a total value of nearly USD $9 billion.
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The name "diamond" derives from the ancient Greek adamas (αδάμας; "invincible"). They have been treasured as gems since their use as religious icons in India at least 2,500 years ago—and usage in drill bits and engraving tools also dates to early human history. Popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, and growth in the world economy. They are commonly judged by the "four Cs": carat, clarity, color, and cut. Although nearly four times the mass of natural diamonds are produced as synthetic diamond each year, the vast majority of synthetic diamond production remains small, imperfect diamonds suitable only for industrial-grade use.
Most natural 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 generally mined from volcanic pipes, which are deep in the Earth where the high pressure and temperature enables the formation of the crystals. The mining and distribution of natural diamonds are subjects of frequent controversy—such as with concerns over the sale of conflict diamonds by African paramilitary groups. There are also allegations that the De Beers Group misuses its dominance in the industry to control supply and manipulate price via monopolistic practices, although in recent years the company's market share has dropped to below 50%.
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Diamond is a transparent crystal of pure carbon consisting of tetrahedrally bonded carbon atoms. Humans have been able to adapt diamonds for many uses because of the material's exceptional physical characteristics. Most notable among these properties are the extreme hardness of diamond, its high dispersion index, and high thermal conductivity. These properties form the basis for most modern applications of diamond.
Diamonds typically crystallize in the face-centered cubic crystal system and consist of tetrahedrally bonded carbon atoms. The unit cell of diamond has a two atom basis at (0,0,0) and (1/4,1/4,1/4), which means half of the atoms are at lattice points and the other half are offset by (1/4,1/4,1/4), where 1 is the length of a side of the unit cell.
The tetrahedral arrangement of atoms in a diamond crystal is the source of many of diamond's properties. Graphite, another allotrope of carbon, has a
rhombohedra crystal structure and as a result shows dramatically different physical characteristics — contrary to diamond, graphite is a very soft, dark gray, opaque mineral. Other elements of the carbon group such as silicon have forms analogous to diamond.
Lonsdaleite is a polymorph of diamond (and a distinct mineral species) that crystallizes with hexagonal symmetry; it is rarely found in nature, but is characteristic of synthetic diamonds. A cryptocrystalline variety of diamond is called carbonado. A colorless, grey or black diamond with a tiny radial structure is a spherulite.
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Diamond is the hardest known naturally occurring material, scoring 10 on the relative Mohs scale of mineral hardness and having an absolute hardness value of between 167 and 231 gigapascals in various tests. Diamond's hardness has been known since antiquity, and is the source of its name. However, aggregated diamond nanorods, an allotrope of carbon first synthesized in 2005, are now believed to be even harder than diamond.The hardest diamonds in the world are diamonds from the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semi perfect octahedral, and are used to polish other diamonds. Their hardness is considered to be a product of the crystal growth form, which is single stage growth crystal. 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 (Taylor et al. 1990).
Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. It is one of the most known and most useful of more than 3,000 known minerals. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial adaptations of this ability include diamond-tipped drill bits and saws, or use of diamond powder as an abrasive. Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical insulators. Industrial-grade diamonds are either unsuitable for use as gems or synthetically produced, which lowers their price and makes their use economically feasible. Industrial applications, especially as drill bits and engraving tools, also date to ancient times.
The hardness of diamonds also contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well, keeping its luster over long periods of time. 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 an engagement ring or wedding ring, which are often worn every day.
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Unlike hardness, which only denotes resistance to scratching, diamond's toughness is only fair to good. Toughness relates to a material's ability to resist breakage from forceful impact. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond is therefore more fragile in some orientations than others.Diamonds occur in a variety of transparent hues — colorless, white, steel, blue, yellow, orange, red, green, pink, brown—or colored black. Diamonds with a detectable hue to them are known as colored diamonds. If the color is strong enough, a stone may be referred to as a fancy colored diamond by the trade. 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. The most common impurity, nitrogen, causes a slight to strong yellow coloration depending upon the type and concentration of nitrogen present.
At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (ΔG = −2.99 kJ / mol). Diamonds will burn at approximately 800 degrees Celsius, providing that enough oxygen is available. This was shown in the late 18th century, and previously described during Roman times. However, owing to a very large kinetic energy barrier, diamonds are metastable; under normal conditions, it would take an extremely long time (possibly more than the age of the Universe) for diamond to decay into graphite.
Diamonds exhibit a high dispersion of visible light. This strong ability to split white light into its component colors is an important aspect of diamond's attraction as a gemstone, giving it impressive prismatic action that results in so-called fire in a well-cut stone. The luster of a diamond, a characterization of how light interacts with the surface of a crystal, is brilliant and is described as adamantine, which simply means diamond-like. This is owed to their high refractive index of 2.417 (at 589.3 nm), which causes total internal reflection to occur. Some diamonds exhibit fluorescence of various colors (predominately blue) under long wave ultraviolet light. Nearly all diamonds fluoresce bluish-white, yellow or green under X-rays and this property is used extensively in mining to separate the fluorescing diamond from the non-fluorescing rock. Most diamonds show no fluorescence although colored diamonds show a wider range of fluorescence than the blue fluorescence normally observed in clear diamonds
Except for most blue diamonds, which are semiconductors, diamonds are good electrical insulators. Blue diamonds owe their semi conductive property to boron impurities, which act as a doping agent and cause p-type semiconductor behavior. Blue diamonds which are not boron-doped, such as those recently recovered from the Argyle diamond mine in Australia that owe their color to an overabundance of hydrogen atoms, are not semiconductors.
Unlike most electrical insulators, diamond is a good conductor of heat because of the strong covalent bonding within the crystal. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductivity. Specially purified synthetic diamond has the highest thermal conductivity (2000–2500 W/(m·K), five times more than copper) of any known solid at room temperature. Because diamond has such high thermal conductance it is already used in semiconductor manufacture to prevent silicon and other semi conducting materials from overheating.
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