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 Diamond is the hardest known
natural material (second-hardest material to ultrahard fullerite), and
is the most expensive 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 be scratched only by other diamonds, Borazon, ultrahard
fullerite, or aggregated diamond nanorods, 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. About 100 tons are synthesized annually.[1]
The name “diamond” derives from the ancient Greek adamas (αδάμας;
“invincible”). They have been treasured as gemstones 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, growth in the world
economy, and innovative and successful advertising campaigns. They are
commonly judged by the “four Cs”: carat, clarity, color, and cut.
Although synthetic diamonds are produced each year at nearly four times
the rate of natural diamonds, the vast majority of synthetic diamonds
produced are small imperfect diamonds suitable only for industrial-grade
use.
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 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 60%.
Crystal structure :
Diamonds typically crystallize in the face-centered cubic crystal system
(space group ) 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. Diamond's density is 3.52 g·cm−3.
The tetrahedral arrangement of atoms is the source of many of diamond’s
properties. The carbon atoms in Graphite, the other major allotrope of
carbon, display a different (nontetrahedral) connectivity and as a
result shows dramatically different physical characteristics: graphite
is a soft, dark gray, opaque mineral. Other elements of the carbon group
such as silicon crystalize like 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 to black diamond with
a tiny radial structure is a spherulite.
Hardness :
Diamond is the hardest natural material known, scoring 10 on the
relative Mohs scale of mineral hardness and having an absolute hardness
value of between 90, 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.[2]
The hardest diamonds in the world are from 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 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.
Toughness :
The toughness of natural diamond has been measured as 3.4 [3] , which is
good compared to other gemstones, but poor compared to most engineering
materials. 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.
Color :
Diamonds can occur in nearly any color, though yellow and brown are by
far the most common. "Black" diamonds are not truly black, but rather
contain numerous dark inclusions that give the gems their dark
appearance. Diamonds with a detectable hue other than yellow or brown
are known as coloured diamonds. If the colour 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 intense
yellow coloration depending upon the type and concentration of nitrogen
present. The color scale for colorless diamonds goes from 'D'
(colorless) to 'Z' (dark yellow).
Formation :
Diamonds are formed by prolonged exposure of carbon bearing materials to
high pressure and temperature. On Earth, the formation of diamonds is
possible because there are regions deep within the Earth that are at a
high enough pressure and temperature that the formation of diamonds is
thermodynamically favorable. Under continental crust, diamonds form
starting at depths of about 150 kilometers (90 miles), where pressure is
roughly 5 gigapascals and the temperature is around 1200 degrees Celsius
(2200 degrees Fahrenheit). Diamond formation under oceanic crust takes
place at greater depths because of higher temperatures, which require
higher pressure for diamond formation. Long periods of exposure to these
high pressures and temperatures allow diamond crystals to grow larger.
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 carbons have
measurably different 13C:12C ratios. Diamonds that have come to the
Earth's surface are generally very old, ranging from under 1 billion to
3.3 billion years old. 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. This is all due to the
conditions in which they form. Diamonds (especially those with rounded
crystal faces) are commonly found coated in nyf, an opaque gum-like
skin.[5]
Diamonds can also form in other natural high-pressure, high-temperature
events. Very small diamonds, known as microdiamonds or nanodiamonds,
have been found in impact craters where meteors strike the Earth and
create shock zones of high pressure and temperature where diamond
formation can occur. Microdiamonds are now used as one indicator of
ancient meteorite impact sites.
Gemological characteristics :
The use of diamonds as gemstones of decorative value is the most
familiar use to most people today, and is also the earliest use, with
decorative use of diamonds stretching back into antiquity. The
dispersion of white light into a rainbow of colors, known in the trade
as fire, is the other primary characteristic of gem diamonds, and has
been highly prized throughout history. Over time, especially since
around 1900, experts in the field of gemology have developed methods of
characterizing 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, clarity, color, and cut.
Most gem diamonds are traded on the wholesale market based on single
values for each of the four Cs; for example knowing that a diamond is
rated as 1.5 carats, VS2 clarity, F color, excellent cut, is enough to
reasonably establish an expected price range. More detailed information
from within each characteristic can then be used to determine actual
market value for individual stones. Consumers who purchase individual
diamonds are often advised to use the four Cs to pick the diamond that
is "right" for them; to these is sometimes added the "fifth C" of
credentials.
Other characteristics not described by the four Cs can and do influence
the value or appearance of a gem diamond. These characteristics include
physical characteristics such as the presence of fluorescence, as well
as data on a diamond's history including its source and which
gemological institute performed evaluation services on the diamond.
Cleanliness also dramatically affects a diamond's beauty.
Sources : Internet
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