Diamond
In
mineralogy,
diamond (from the ancient
Greek αδάμας –
adámas "unbreakable") is a
metastable allotrope of carbon, where the carbon
atoms are arranged in a variation of the
face-centered cubic crystal structure called a
diamond lattice. Diamond is less
stable than
graphite, but the conversion rate from diamond to graphite is negligible at
standard conditions. Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong
covalent bonding between its atoms. In particular, diamond has the highest
hardness and
thermal conductivity
of any bulk material. Those properties determine the major industrial
application of diamond in cutting and polishing tools and the scientific
applications in
diamond knives and
diamond anvil cells.
Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as
boron and
nitrogen.
Small amounts of defects or impurities (about one per million of
lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (
lattice defects), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high
optical dispersion (ability to disperse light of different colors).
Most natural diamonds are formed at high temperature and pressure at
depths of 140 to 190 kilometers (87 to 118 mi) in the Earth's
mantle.
Carbon-containing minerals provide the carbon source, and the growth
occurs over periods from 1 billion to 3.3 billion years (25% to 75% of
the
age of the Earth). Diamonds are brought close to the Earth′s surface through deep
volcanic eruptions by a
magma, which cools into
igneous rocks known as
kimberlites and
lamproites. Diamonds can also be produced synthetically in a
high-pressure high-temperature
process which approximately simulates the conditions in the Earth's
mantle. An alternative, and completely different growth technique is
chemical vapor deposition (CVD). Several non-diamond materials, which include
cubic zirconia and
silicon carbide and are often called
diamond simulants, resemble diamond in appearance and many properties. Special
gemological techniques have been developed to distinguish natural and
synthetic diamonds and diamond simulants.
History
The name
diamond is derived from the ancient Greek
αδάμας (adámas), "proper", "unalterable", "unbreakable", "untamed", from
ἀ- (a-), "un-" +
δαμάω (
damáō), "I overpower", "I tame".
[3] 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.
[4]
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.
[5][6]
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.
[7]
In 1772,
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.
[8] Later in 1797,
Smithson Tennant repeated and expanded that experiment.
[9]
By demonstrating that burning diamond and graphite releases the same
amount of gas he established the chemical equivalence of these
substances.
[10]
The most familiar use of diamonds today is as gemstones used for
adornment, a use which dates back into antiquity. The
dispersion of white light into
spectral colors
is the primary gemological characteristic of gem diamonds. In the 20th
century, experts in gemology have 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
carat,
cut,
color, and
clarity.
[11] A large, flawless diamond is known as a
paragon.
Natural history
The formation of natural diamond requires very specific
conditions—exposure of carbon-bearing materials to high pressure,
ranging approximately between 45 and 60
kilobars (4.5 and 6
GPa),
but at a comparatively low temperature range between approximately 900
and 1,300 °C (1,650 and 2,370 °F). These conditions are met in two
places on Earth; in the
lithospheric mantle below relatively stable
continental plates, and at the site of a meteorite strike.
[12]
Formation in cratons
One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching
The conditions for diamond formation to happen in the lithospheric
mantle occur at considerable depth corresponding to the requirements of
temperature and pressure. These depths are estimated between 140 and 190
kilometers (87 and 118 mi) though occasionally diamonds have
crystallized at depths about 300 kilometers (190 mi).
[13] 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. 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. Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
[13]
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. These two different source of carbon 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.
[13]
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. Sometimes they are found grown together or form double
"twinned" crystals at the surfaces of the octahedron. These different
shapes and habits of some diamonds result from differing external
circumstances. Diamonds (especially those with rounded crystal faces)
are commonly found coated in
nyf, an opaque gum-like skin.
[14]
Space diamonds
Primitive interstellar meteorites were found to contain carbon possibly in the form of diamond (Lewis et al. 1987).
[15] Not all diamonds found on Earth originated here. A type of diamond called
carbonado
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. These diamonds may have formed in the intrastellar
environment, but as of 2008, there was no scientific consensus on how
carbonado diamonds originated.
[16][17]
Diamonds can also form under other naturally occurring high-pressure
conditions. Very small diamonds of micrometer and nanometer sizes, known
as
microdiamonds or
nanodiamonds respectively, 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
an indicator of ancient impact craters.
[12] Popigai crater in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.
[18]
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 (4.7
×10
14 km) away in the constellation
Centaurus. A news release from the
Harvard-Smithsonian Center for Astrophysics described the 2,500-mile (4,000 km)-wide stellar core as a
diamond.
[19] It was referred to as
Lucy, after the Beatles' song "Lucy in the Sky With Diamonds".
[20][21]
Transport from mantle
Schematic diagram of a volcanic pipe
Diamond-bearing rock is carried from the mantle to the Earth's
surface by deep-origin volcanic eruptions. The magma for such a volcano
must originate at a depth where diamonds can be formed
[13]—150 km
(93 mi) or more (three times or more the depth of source magma for most
volcanoes). This is a relatively rare occurrence. These typically small
surface volcanic craters extend downward in formations known as
volcanic pipes.
[13]
The pipes contain material that was transported toward the surface by
volcanic action, but was not ejected before the volcanic activity
ceased. During eruption these pipes are open to the surface, resulting
in open circulation; many
xenoliths
of surface rock and even wood and fossils are found in volcanic pipes.
Diamond-bearing volcanic pipes are closely related to the oldest,
coolest regions of
continental crust
(cratons). This is because cratons are very thick, and their
lithospheric mantle extends to great enough depth that diamonds are
stable. Not all pipes contain diamonds, and even fewer contain enough
diamonds to make mining economically viable.
[13] Diamonds are very rare
[22]
because most of the crust is too thin to permit diamond
crystallization, whereas most of the mantle has relatively little
carbon.
The magma in volcanic pipes is usually one of two characteristic
types, which cool into igneous rock known as either kimberlite or
lamproite.
[13]
The magma itself does not contain diamond; instead, it acts as an
elevator that carries deep-formed rocks (xenoliths), minerals (
xenocrysts), and fluids upward. These rocks are characteristically rich in
magnesium-bearing
olivine,
pyroxene, and
amphibole minerals
[13] which are often altered to
serpentine by heat and fluids during and after eruption. Certain
indicator minerals
typically occur within diamantiferous kimberlites and are used as
mineralogical tracers by prospectors, who follow the indicator trail
back to the volcanic pipe which may contain diamonds. These minerals are
rich in
chromium (Cr) or
titanium (Ti), elements which impart bright colors to the minerals. The most common indicator minerals are chromium
garnets (usually bright red chromium-
pyrope, and occasionally green ugrandite-series garnets), eclogitic garnets, orange titanium-pyrope, red high-chromium
spinels, dark
chromite, bright green chromium-
diopside, glassy green olivine, black
picroilmenite, and
magnetite. Kimberlite deposits are known as
blue ground for the deeper serpentinized part of the deposits, or as
yellow ground for the near surface
smectite clay and carbonate
weathered and
oxidized portion.
[13]
Once diamonds have been transported to the surface by magma in a
volcanic pipe, they may erode out and be distributed over a large area. A
volcanic pipe containing diamonds is known as a
primary source of diamonds.
Secondary sources
of diamonds include all areas where a significant number of diamonds
have been eroded out of their kimberlite or lamproite matrix, and
accumulated because of water or wind action. These include
alluvial
deposits and deposits along existing and ancient shorelines, where
loose diamonds tend to accumulate because of their size and density.
Diamonds have also rarely been found in deposits left behind by glaciers
(notably in
Wisconsin and
Indiana); in contrast to alluvial deposits, glacial deposits are minor and are therefore not viable commercial sources of diamond.
[13]
Material properties
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 in a covalent network lattice (
sp3) that crystallizes into the
diamond lattice which is a variation of the
face centered cubic
structure. Diamonds have been adapted for many uses because of the
material's exceptional physical characteristics. Most notable are its
extreme hardness and thermal conductivity (900–
2,320 W·m−1·K−1),
[23] as well as wide
bandgap and high optical dispersion.
[24] Above
1,700 °C (
1,973 K /
3,583 °F) in
vacuum or oxygen-free atmosphere, diamond converts to graphite; in air, transformation starts at ~
700 °C.
[25] Diamond's ignition point is 720 –
800 °C in oxygen and 850 –
1,000 °C in air.
[26] Naturally occurring diamonds have a density ranging from 3.15–
3.53 g/cm3, with pure diamond close to
3.52 g/cm3.
[1]
The chemical bonds that hold the carbon atoms in diamonds together are
weaker than those in graphite. In diamonds, the bonds form an inflexible
three-dimensional lattice, whereas in graphite, the atoms are tightly
bonded into sheets, which can slide easily over one another, making the
overall structure weaker.
[27]
Hardness
Diamond is the hardest known natural material on the
Mohs scale of mineral hardness,
where hardness is defined as resistance to scratching and is graded
between 1 (softest) and 10 (hardest). Diamond has a hardness of 10
(hardest) on this scale.
[28] Diamond's hardness has been known since antiquity, and is the source of its name.
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).
[29] 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 extreme hardness of diamond in certain orientations makes it useful
in materials science, as in this pyramidal diamond embedded in the
working surface of a
Vickers hardness tester.
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.
[20]
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 7.5–10
MPa·m
1/2.
[30][31]
This value 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.
[32] "Impact toughness" is one of the main indexes to measure the quality of synthetic industrial diamonds.
[26]
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.
[33]
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.
[33]
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.
[34][35]
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.
[26] 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.
[36]
Chemical stability
Diamonds are not very reactive. Under room temperature diamonds do
not react with any chemical reagents including strong acids and bases. A
diamond's surface can only be oxidized a little by just a few oxidants
[which?] at high temperature (below
1,000 °C). Therefore, acids and bases can be used to refine synthetic diamonds.
[26]
Color
Main article:
Diamond color
Diamond has a wide
bandgap of
5.5 eV corresponding to the deep
ultraviolet
wavelength of 225 nanometers. This means 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 Ni and Co,
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 Ni
[37] and even less for Co. Virtually any element can be introduced to diamond by ion implantation.
[38]
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.
[24]
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
[39] and perhaps pink and red diamonds.
[40] In order of rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red.
[32] "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.
[32] 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).
Diamonds of a different color, such as blue, are called
fancy colored diamonds, and fall under a different grading scale.
[32]
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.
[41]
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 euro or US$9.5 million at the
time).
[42]
That record was however beaten the same year: a 5-carat (1.0 g) vivid
pink diamond was sold for $10.8 million in Hong Kong on December 1,
2009.
[43]
Identification
Diamonds can be identified by their high thermal conductivity. Their high
refractive index
is also indicative, but other materials have similar refractivity.
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.
[28]
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;
[10]
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'.
[44]
Industry
The diamond industry can be separated into two distinct categories:
one dealing with gem-grade diamonds and another for industrial-grade
diamonds. Both markets value diamonds differently.
Gem-grade diamonds
A large trade in gem-grade diamonds exists. Unlike other commodities,
such as most precious metals, there is a substantial mark-up in the
retail sale of gem diamonds.
[45]
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.
[46] 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,
Tel Aviv, and Amsterdam. A single company –
De Beers – controls a significant proportion of the trade in diamonds.
[47] They are based in
Johannesburg,
South Africa and London, England. 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 operated by De Beers that can
produce between 12,500,000 carats (2,500 kg) to 15,000,000 carats
(3,000 kg) of diamonds per year,
[48])
whereas secondary alluvial diamond deposits 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.
[49] This makes Antwerp a de facto "world diamond capital".
[50] Another important diamond center is
New York City, where almost 80% of the world's diamonds are sold, including auction sales.
[49]
The DeBeers company, as the world's largest diamond miner holds a
dominant position in the industry, and has done so since soon after its
founding in 1888 by the British imperialist
Cecil Rhodes. De Beers owns or controls a significant portion of the world's rough 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,
[51] but by 2001–2009 the figure had decreased to around 45%,
[52] and by 2013 the company's market share had further decreased to around 38% in value terms and even less by volume.
[53] De Beers sold off the vast majority of its diamond stockpile in the late 1990s – early 2000s
[54] and the remainder largely represents working stock (diamonds that are being sorted before sale).
[55] This was well documented in the press
[56] 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.
[57]
As of January 2011, De Beers states that it only sells diamonds from
the following four countries: Botswana, Namibia, South Africa and
Canada.
[58] Alrosa had to suspend their sales in October 2008 due to the
global energy crisis,
[59] but the company reported that it had resumed selling rough diamonds on the open market by October 2009.
[60] Apart from Alrosa, other important diamond mining companies include
BHP Billiton, which is the world's largest mining company;
[61] Rio Tinto Group, the owner of Argyle (100%),
Diavik (60%), and
Murowa (78%) diamond mines;
[62] and
Petra Diamonds, the owner of several major diamond mines in Africa.
Diamond polisher in Amsterdam
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 USA, Europe and Asia.
[32] 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).
[63]
The cutting and polishing of rough diamonds is a specialized skill that
is concentrated in a limited number of locations worldwide.
[63] 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.
[63] 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.
[49]
Diamonds which have been prepared as gemstones are sold on diamond exchanges called
bourses. There are 28 registered diamond bourses in the world.
[64]
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 Group, 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.
[65]
Cutting
The
Darya-I-Nur Diamond—an example of unusual diamond cut and jewelry arrangement
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%.
[66]
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.
[67]
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.
- The diamond can be split by a single, well calculated blow of a
hammer to a pointed tool, which is quick, but risky. Alternatively, it
can be cut with a diamond saw, which is a more reliable but tedious procedure.[67][68]
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.
[69]
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.
[28]
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 also advertising its
competitors' diamond products as well.
[70]
De Beers' market share dipped temporarily to 2nd 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.
[71]
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.
[71] The campaign was perhaps best captured by the slogan "a diamond is forever".
[7] This slogan is now being used by De Beers Diamond Jewelers,
[72] 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.
[73][74]
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;
[75] 80% of Argyle diamonds are brown.
[76]
Industrial-grade diamonds
A
scalpel with synthetic diamond blade
Close-up photograph of an
angle grinder blade with tiny diamonds shown embedded in the metal
A diamond knife blade used for cutting ultrathin sections (typically 70 to 350 nm for transmission
electron microscopy.
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. 80% of mined diamonds (equal to about
135,000,000 carats (27,000 kg) annually), are unsuitable for use as
gemstones, and used industrially.
[77]
In addition to mined diamonds, synthetic diamonds found industrial
applications almost immediately after their invention in the 1950s;
another 570,000,000 carats (114,000 kg) of synthetic diamond is produced
annually for industrial use. Approximately 90% of diamond
grinding grit is currently of synthetic origin.
[78]
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.
[79]
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, known as bort, with more
flaws and poorer color than gems, are used for such purposes.
[80] 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.
[81]
Specialized applications include use in laboratories as containment for
high pressure experiments (see
diamond anvil cell), high-performance
bearings, and limited use in specialized
windows.
[79]
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.
[82]
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.
[83]
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.
[78]
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.
[84]
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,
[66]
the separation was done with grease belts; diamonds have a stronger
tendency to stick to grease than the other minerals in the ore.
[32]
Historically, diamonds were found only in
alluvial deposits in
Guntur and
Krishna district of the
Krishna River delta in
Southern India.
[85] India led the world in diamond production from the time of their discovery in approximately the 9th century BC
[4][86]
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.
[4] Currently, one of the most prominent Indian mines is located at
Panna.
[87]
Diamond extraction from primary deposits (kimberlites and lamproites) started in the 1870s after the discovery of the
Diamond Fields in South Africa.
[88]
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.
[89]
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.
[89]
In the U.S., diamonds have been found in
Arkansas,
Colorado, Wyoming, and
Montana.
[90][91] 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.
[91]
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 Congo.
[92] In 2005, Russia produced almost one-fifth of the global diamond output, reports 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.
[90][93] There are also commercial deposits being actively mined in the
Northwest Territories of Canada and Brazil.
[78] Diamond prospectors continue to search the globe for diamond-bearing kimberlite and lamproite pipes.
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.
[84]
Major diamond trading corporations continue to fund and fuel these
conflicts by doing business with armed groups. 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.
[94]
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. Conflict diamonds
constitute 2–3% of all diamonds traded.
[95]
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".
[94]
The Canadian Government has set up a body known as Canadian Diamond Code of Conduct
[96]
to help authenticate Canadian diamonds. This is a stringent tracking
system of diamonds and helps protect the "conflict free" label of
Canadian diamonds.
[97]
Synthetics, simulants, and enhancements
Synthetics
Synthetic diamonds of various colors grown by the high-pressure high-temperature technique
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.
[13]
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.
[98]
The majority of commercially available synthetic diamonds are yellow
and are produced by so-called High Pressure High Temperature (
HPHT) processes.
[99]
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.
[100]
Colorless gem cut from diamond grown by chemical vapor deposition
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 to chemically active
radicals in a
plasma ignited by
microwaves,
hot filament,
arc discharge,
welding torch or
laser.
[101] This method is mostly used for coatings, but can also produce single crystals several millimeters in size (see picture).
[83]
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.
[98][102]
The cost of mining a natural colorless diamond runs about $40 to $60
per carat, and the cost to produce a synthetic, gem-quality colorless
diamond is about $2,500 per carat.
[98]
However, a purchaser is more likely to encounter a synthetic when
looking for a fancy-colored diamond because nearly all synthetic
diamonds are fancy-colored, while only 0.01% of natural diamonds are.
[103]
Simulants
Gem-cut synthetic silicon carbide set in a ring
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.
[104]
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.
[105]
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.
[106]
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 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 2–3 seconds.
[107]
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.
[108]
"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.
[109]
Laboratories use techniques such as spectroscopy, microscopy and
luminescence under shortwave ultraviolet light to determine a diamond's
origin.
[108] 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.
[110]
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
[111]
Diamond Spotter. Stones in the D-Z color range can be examined through
the DiamondSure UV/visible spectrometer, a tool developed by De Beers.
[109]
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) and Alpha Diamond
Analyzer (Bruker / HRD Antwerp).
Stolen diamonds
Occasionally large thefts of diamonds take place. In February 2013
armed robbers carried out a raid at Brussels Airport and escaped with
gems estimated to be worth $50m (£32m; 37m euros). The gang broke
through a perimeter fence and raided the cargo hold of a Swiss-bound
plane. The gang have since been arrested and large amounts of cash and
diamonds recovered.
[112]
The identification of stolen diamonds presents a set of difficult
problems. Rough diamonds will have a distinctive shape depending on
whether their source is a mine or from an alluvial environment such as a
beach or river - alluvial diamonds have smoother surfaces than those
that have been mined. Determining the provenance of cut and polished
stones is much more complex.
The
Kimberley Process
was developed to monitor the trade in rough diamonds and prevent their
being used to fund violence. Before exporting, rough diamonds are
certificated by the government of the country of origin. Some countries,
such as Venezuela, are not party to the agreement. The Kimberley
Process does not apply to local sales of rough diamonds within a
country.
Diamonds may be etched by laser with marks invisible to the naked eye.
Lazare Kaplan, a US-based company, developed this method. However, whatever is marked on a diamond can readily be removed.
