This article is about the mineral. For the gemstone, see Diamond (gemstone). For other uses, including the shape ◊, see Diamond (disambiguation).
Diamond
A scattering of round-brilliant cut diamonds shows off the many reflecting facets.
General
Category
Native Minerals
Chemical formula
C
Identification
Molecular Weight
12.01 u
Color
Typically yellow, brown or gray to colorless. Less often in blue, green, black, translucent white, pink, violet, orange, purple and red.[1]
Crystal habit
Octahedral
Crystal system
Isometric-Hexoctahedral (Cubic)
Cleavage
111 (perfect in four directions)
Fracture
Conchoidal (shell-like)
Mohs Scale hardness
10[1]
Luster
Adamantine[1]
Polish luster
Adamantine[1]
Refractive index
2.4175–2.4178
Optical Properties
Singly Refractive[1]
Birefringence
None[1]
Dispersion
0.044[1]
Pleochroism
None[1]
Ultraviolet fluorescence
Colorless to yellowish stones; inert to strong in long wave, and typically blue. Weaker in short wave.[1]
Absorption spectra
In pale yellow stones a 415.5 nm line is typical. Irradiated and annealed diamonds often show a line around 594 nm when cooled to low temperatures.[1]
Streak
White
Specific gravity
3.52 (± 0.01)[1]
Density
3.5-3.53 g/cm³
Diaphaneity
Transparent to subtransparent to translucent
In mineralogy, diamond (from the ancient Greek ἀδάμας, adámas) is the allotrope of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. After graphite, diamond is the second most stable form of carbon. Its hardness and high dispersion of light make it useful for industrial applications and jewelry. It is the hardest known naturally occurring mineral. 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.[2] Presently, only aggregated diamond nanorods, a material created using ultrahard fullerite (C60) is confirmed to be harder, although other substances such as cubic boron nitride, rhenium diboride and ultrahard fullerite itself are comparable.
Diamonds are specifically renowned as a material with superlative physical qualities; they make excellent abrasives because they can be scratched only by other diamonds, borazon, ultrahard fullerite, rhenium diboride, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain their lustre. Approximately 130 million carats (26,000 kg (57,000 lb)) are mined annually, with a total value of nearly USD $9 billion, and about 100,000 kg (220,000 lb) are synthesized annually.[3]
The name diamond is derived from the ancient Greek ἀδάμας (adámas), "unbreakable, untamed", from ἀ- (a-), "un-" + δαμάω (damáō), "to overpower, to tame"[4]. They have been treasured as gemstones since their use as religious icons in ancient India and usage in engraving tools also dates to early human history.[5][6] 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. They are commonly judged by the “four Cs”: carat, clarity, color, and cut.
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 with concerns over the sale of conflict diamonds (aka blood diamonds) by African paramilitary groups.
Contents[hide]
1 Material properties
1.1 Hardness
1.2 Electrical conductivity
1.3 Toughness
1.4 Color
1.5 Identification
2 Natural history
2.1 Formation
2.1.1 Diamonds formed in cratons
2.1.2 Diamonds and meteorite impact craters
2.1.3 Extraterrestrial diamonds
2.2 Surfacing
3 History and gemological characteristics
4 The diamond industry
4.1 Gem diamond industry
4.2 Industrial diamond industry
4.3 Diamond supply chain
4.3.1 Mining, sources and production
4.3.2 "Blood" diamonds
4.3.3 Distribution
4.4 Crater of Diamonds State Park
5 Synthetics, simulants, and enhancements
6 See also
7 Notes
8 References
9 External links
//
[edit] Material properties
Main article: Material properties of diamond
See also: Crystallographic defects in diamond
Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.
A diamond is a transparent crystal of tetrahedrally bonded carbon atoms and crystallizes into the face centered cubic diamond lattice structure. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness, its high dispersion index, and extremely high thermal conductivity (900 – 2320 W/m K). Above 1700 °C (1973 K / 3583 °F), diamond is converted to graphite.[7] Naturally occurring diamonds have a density ranging from 3.15 to 3.53 g/cm³, with very pure diamond typically extremely close to 3.52 g/cm³.
[edit] Hardness
Diamond is the hardest natural material known, where hardness is defined as resistance to scratching.[8] Diamond has a hardness of 10 (hardest) on Mohs scale of mineral hardness.[9] Diamond's hardness has been known since antiquity, and is the source of its name.
The hardest diamonds in the world are from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. They were called can-ni-fare (cannot be cut) by the Cutters in Antwerpt, when they started to arrive in quantity, from Australia in the 1870s. These diamonds are generally small, perfect to semiperfect octahedra, 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.[10]
The hardness of diamonds 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.
Industrial use of diamonds has historically been associated with their hardness; this property 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 adaptations of this ability include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds, known as bort, with more flaws and poorer colour 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 when compared to alternatives.
[edit] 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 other diamonds, which are excellent electrical insulators.[9] The conductivity and blue color originate from the 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.[11] This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.
[edit] Toughness
Toughness relates to a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 3.4 MN m-3/2,[12] which is good compared to other gemstones, but poor compared to most engineering materials. 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, prior to faceting.
[edit] Color
Main article: Diamond color
Brown colored diamonds at the National Museum of Natural History
Gem quality diamond may be colorless or occur in any hue including the non-spectral hues of gray, brown and black. Diamond is the only gemstone composed of a single element, carbon. The diamond crystal lattice is exceptionally strong and only atoms of nitrogen, boron, hydrogen, phosphorus and maybe beryllium can be introduced into diamond during the growth at significant concentrations. Transition metals Ni and Co, which are commonly used for growth of synthetic diamond by the high-pressure high-temperature techniques, have been detected in diamond as individual atoms, however the maximum concentration is 0.01% for Ni[13] and even much less for Co. Note however, that virtually any element can be introduced in diamond by ion implantation.
Nitrogen is the smallest and by far the most common impurity found in gem diamonds. Nitrogen is responsible for the yellow and brown in diamonds. Boron is responsible for the gray blue colors. Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds; and physical deformation of the diamond crystal known as plastic deformation. Plastic deformation is the cause of color in some brown[14] and perhaps pink and red diamonds.[15] In order of rarity, colorless diamond, by far the most common, is followed by blue, green, black, translucent white, pink, violet, orange, purple and red, though yellow and brown are by far the most common colors.[9] "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.[9] 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).
In 2008, the Wittelsbach Diamond, a 35.56 carat blue diamond once belonging to the King of Spain, fetched over $24M US at a Christie's auction. The blue hue was a result of trace amounts of boron in the stone's crystal structure.[16]
[edit] Identification
Diamonds can be identified by their high thermal conductivity. Their high refractive index is also indicative, but other materials have similar refractivity. Diamonds do cut glass, but other materials above glass on Mohs scale such as quartz do also. Diamonds easily scratch other diamonds, but this damages both diamonds.
[edit] Natural history
[edit] Formation
The formation of natural diamond requires very specific conditions. Diamond formation requires exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars,[17] but at a comparatively low temperature range between approximately 1652–2372 °F (900–1300 °C).[17] These conditions are known to be met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.
[edit] Diamonds formed in cratons
The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the aforementioned requirements of temperature and pressure. These depths are estimated to be in between 140–190 kilometers (90–120 miles)[17][9] though occasionally diamonds have crystallized at depths of 300-400 km (180-250 miles) as well.[18] The rate at which temperature changes with increasing depth into the Earth varies greatly in different parts of the Earth. In particular, under oceanic plates the temperature rises more quickly with depth, beyond the range required for diamond formation at the depth required.[17] The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist.[17] Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
The slightly misshapen octahedral shape of this rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit.
Through studies of carbon isotope ratios (similar to the methodology used in carbon dating, except with the stable isotopes C-12 and C-13), it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth's mantle. In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth's crust through subduction (see plate tectonics) before transforming into diamond.[9] These two different source carbons have measurably different 13C:12C ratios. Diamonds that have come to the Earth's surface are generally quite old, ranging from under 1 billion to 3.3 billion years old. This is 22% to 73% of the age of the Earth.
Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles or maccles. 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. Sometimes they are found grown together or form double "twinned" crystals grown together at the surfaces of the octahedron. These different shapes and habits of the diamonds result from differing external circumstances. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin.[19]
[edit] Diamonds and meteorite impact craters
Diamonds can also form in other natural high-pressure events. Very small diamonds, known as microdiamonds or nanodiamonds, have been found in meteorite impact craters. Such impact events create shock zones of high pressure and temperature suitable for diamond formation. Impact-type microdiamonds can be used as one indicator of ancient impact craters.[9]
[edit] Extraterrestrial diamonds
Not all diamonds found on earth originated here. A type of diamond called carbonado diamond that is found in South America and Africa may have been deposited there via an asteroid impact (not formed from the impact) about 3 billion years ago.[20][21] These diamonds may have formed in the intrastellar environment, but as of 2008, there was no scientific consensus on how carbonado diamonds originated.
Presolar grains in many meteorites found on earth contain nanodiamonds of extraterrestrial origin, probably formed in supernovas.
Scientific evidence indicates that white dwarf stars have a core of crystallized carbon and oxygen nuclei. The largest of these found in the universe so far, BPM 37093, is located 50 light years away in the constellation Centaurus. A news release from the Harvard-Smithsonian Center for Astrophysics described the 2,500 mile-wide stellar core as a diamond.[22] It is estimated to be ten billion trillion trillion carats, more or less. It was referred to as Lucy, after the Beatles song "Lucy in the Sky With Diamonds".[23][2]
[edit]
Diamond
A scattering of round-brilliant cut diamonds shows off the many reflecting facets.
General
Category
Native Minerals
Chemical formula
C
Identification
Molecular Weight
12.01 u
Color
Typically yellow, brown or gray to colorless. Less often in blue, green, black, translucent white, pink, violet, orange, purple and red.[1]
Crystal habit
Octahedral
Crystal system
Isometric-Hexoctahedral (Cubic)
Cleavage
111 (perfect in four directions)
Fracture
Conchoidal (shell-like)
Mohs Scale hardness
10[1]
Luster
Adamantine[1]
Polish luster
Adamantine[1]
Refractive index
2.4175–2.4178
Optical Properties
Singly Refractive[1]
Birefringence
None[1]
Dispersion
0.044[1]
Pleochroism
None[1]
Ultraviolet fluorescence
Colorless to yellowish stones; inert to strong in long wave, and typically blue. Weaker in short wave.[1]
Absorption spectra
In pale yellow stones a 415.5 nm line is typical. Irradiated and annealed diamonds often show a line around 594 nm when cooled to low temperatures.[1]
Streak
White
Specific gravity
3.52 (± 0.01)[1]
Density
3.5-3.53 g/cm³
Diaphaneity
Transparent to subtransparent to translucent
In mineralogy, diamond (from the ancient Greek ἀδάμας, adámas) is the allotrope of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. After graphite, diamond is the second most stable form of carbon. Its hardness and high dispersion of light make it useful for industrial applications and jewelry. It is the hardest known naturally occurring mineral. 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.[2] Presently, only aggregated diamond nanorods, a material created using ultrahard fullerite (C60) is confirmed to be harder, although other substances such as cubic boron nitride, rhenium diboride and ultrahard fullerite itself are comparable.
Diamonds are specifically renowned as a material with superlative physical qualities; they make excellent abrasives because they can be scratched only by other diamonds, borazon, ultrahard fullerite, rhenium diboride, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain their lustre. Approximately 130 million carats (26,000 kg (57,000 lb)) are mined annually, with a total value of nearly USD $9 billion, and about 100,000 kg (220,000 lb) are synthesized annually.[3]
The name diamond is derived from the ancient Greek ἀδάμας (adámas), "unbreakable, untamed", from ἀ- (a-), "un-" + δαμάω (damáō), "to overpower, to tame"[4]. They have been treasured as gemstones since their use as religious icons in ancient India and usage in engraving tools also dates to early human history.[5][6] 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. They are commonly judged by the “four Cs”: carat, clarity, color, and cut.
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 with concerns over the sale of conflict diamonds (aka blood diamonds) by African paramilitary groups.
Contents[hide]
1 Material properties
1.1 Hardness
1.2 Electrical conductivity
1.3 Toughness
1.4 Color
1.5 Identification
2 Natural history
2.1 Formation
2.1.1 Diamonds formed in cratons
2.1.2 Diamonds and meteorite impact craters
2.1.3 Extraterrestrial diamonds
2.2 Surfacing
3 History and gemological characteristics
4 The diamond industry
4.1 Gem diamond industry
4.2 Industrial diamond industry
4.3 Diamond supply chain
4.3.1 Mining, sources and production
4.3.2 "Blood" diamonds
4.3.3 Distribution
4.4 Crater of Diamonds State Park
5 Synthetics, simulants, and enhancements
6 See also
7 Notes
8 References
9 External links
//
[edit] Material properties
Main article: Material properties of diamond
See also: Crystallographic defects in diamond
Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.
A diamond is a transparent crystal of tetrahedrally bonded carbon atoms and crystallizes into the face centered cubic diamond lattice structure. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness, its high dispersion index, and extremely high thermal conductivity (900 – 2320 W/m K). Above 1700 °C (1973 K / 3583 °F), diamond is converted to graphite.[7] Naturally occurring diamonds have a density ranging from 3.15 to 3.53 g/cm³, with very pure diamond typically extremely close to 3.52 g/cm³.
[edit] Hardness
Diamond is the hardest natural material known, where hardness is defined as resistance to scratching.[8] Diamond has a hardness of 10 (hardest) on Mohs scale of mineral hardness.[9] Diamond's hardness has been known since antiquity, and is the source of its name.
The hardest diamonds in the world are from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. They were called can-ni-fare (cannot be cut) by the Cutters in Antwerpt, when they started to arrive in quantity, from Australia in the 1870s. These diamonds are generally small, perfect to semiperfect octahedra, 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.[10]
The hardness of diamonds 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.
Industrial use of diamonds has historically been associated with their hardness; this property 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 adaptations of this ability include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds, known as bort, with more flaws and poorer colour 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 when compared to alternatives.
[edit] 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 other diamonds, which are excellent electrical insulators.[9] The conductivity and blue color originate from the 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.[11] This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.
[edit] Toughness
Toughness relates to a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 3.4 MN m-3/2,[12] which is good compared to other gemstones, but poor compared to most engineering materials. 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, prior to faceting.
[edit] Color
Main article: Diamond color
Brown colored diamonds at the National Museum of Natural History
Gem quality diamond may be colorless or occur in any hue including the non-spectral hues of gray, brown and black. Diamond is the only gemstone composed of a single element, carbon. The diamond crystal lattice is exceptionally strong and only atoms of nitrogen, boron, hydrogen, phosphorus and maybe beryllium can be introduced into diamond during the growth at significant concentrations. Transition metals Ni and Co, which are commonly used for growth of synthetic diamond by the high-pressure high-temperature techniques, have been detected in diamond as individual atoms, however the maximum concentration is 0.01% for Ni[13] and even much less for Co. Note however, that virtually any element can be introduced in diamond by ion implantation.
Nitrogen is the smallest and by far the most common impurity found in gem diamonds. Nitrogen is responsible for the yellow and brown in diamonds. Boron is responsible for the gray blue colors. Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds; and physical deformation of the diamond crystal known as plastic deformation. Plastic deformation is the cause of color in some brown[14] and perhaps pink and red diamonds.[15] In order of rarity, colorless diamond, by far the most common, is followed by blue, green, black, translucent white, pink, violet, orange, purple and red, though yellow and brown are by far the most common colors.[9] "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.[9] 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).
In 2008, the Wittelsbach Diamond, a 35.56 carat blue diamond once belonging to the King of Spain, fetched over $24M US at a Christie's auction. The blue hue was a result of trace amounts of boron in the stone's crystal structure.[16]
[edit] Identification
Diamonds can be identified by their high thermal conductivity. Their high refractive index is also indicative, but other materials have similar refractivity. Diamonds do cut glass, but other materials above glass on Mohs scale such as quartz do also. Diamonds easily scratch other diamonds, but this damages both diamonds.
[edit] Natural history
[edit] Formation
The formation of natural diamond requires very specific conditions. Diamond formation requires exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars,[17] but at a comparatively low temperature range between approximately 1652–2372 °F (900–1300 °C).[17] These conditions are known to be met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.
[edit] Diamonds formed in cratons
The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the aforementioned requirements of temperature and pressure. These depths are estimated to be in between 140–190 kilometers (90–120 miles)[17][9] though occasionally diamonds have crystallized at depths of 300-400 km (180-250 miles) as well.[18] The rate at which temperature changes with increasing depth into the Earth varies greatly in different parts of the Earth. In particular, under oceanic plates the temperature rises more quickly with depth, beyond the range required for diamond formation at the depth required.[17] The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist.[17] Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
The slightly misshapen octahedral shape of this rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit.
Through studies of carbon isotope ratios (similar to the methodology used in carbon dating, except with the stable isotopes C-12 and C-13), it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth's mantle. In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth's crust through subduction (see plate tectonics) before transforming into diamond.[9] These two different source carbons have measurably different 13C:12C ratios. Diamonds that have come to the Earth's surface are generally quite old, ranging from under 1 billion to 3.3 billion years old. This is 22% to 73% of the age of the Earth.
Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles or maccles. 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. Sometimes they are found grown together or form double "twinned" crystals grown together at the surfaces of the octahedron. These different shapes and habits of the diamonds result from differing external circumstances. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin.[19]
[edit] Diamonds and meteorite impact craters
Diamonds can also form in other natural high-pressure events. Very small diamonds, known as microdiamonds or nanodiamonds, have been found in meteorite impact craters. Such impact events create shock zones of high pressure and temperature suitable for diamond formation. Impact-type microdiamonds can be used as one indicator of ancient impact craters.[9]
[edit] Extraterrestrial diamonds
Not all diamonds found on earth originated here. A type of diamond called carbonado diamond that is found in South America and Africa may have been deposited there via an asteroid impact (not formed from the impact) about 3 billion years ago.[20][21] These diamonds may have formed in the intrastellar environment, but as of 2008, there was no scientific consensus on how carbonado diamonds originated.
Presolar grains in many meteorites found on earth contain nanodiamonds of extraterrestrial origin, probably formed in supernovas.
Scientific evidence indicates that white dwarf stars have a core of crystallized carbon and oxygen nuclei. The largest of these found in the universe so far, BPM 37093, is located 50 light years away in the constellation Centaurus. A news release from the Harvard-Smithsonian Center for Astrophysics described the 2,500 mile-wide stellar core as a diamond.[22] It is estimated to be ten billion trillion trillion carats, more or less. It was referred to as Lucy, after the Beatles song "Lucy in the Sky With Diamonds".[23][2]
[edit]
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