Diamond has remarkable optical characteristics. Because of its
extremely rigid lattice, it can be contaminated by very few types of
impurities, such as
boron and
nitrogen.
Combined with wide transparency, this results in the clear, colorless
appearance of most natural diamonds. 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), which results in its characteristic
luster. Excellent optical and mechanical properties, notably unparalleled hardness and durability, make diamond the most popular
gemstone.
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. Later in 1797,
Smithson Tennant
repeated and expanded that experiment. By demonstrating that burning
diamond and graphite releases the same amount of gas he established the
chemical equivalence of these substances.
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.
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 (1,652 and 2,372 °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.
[10]
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 120 mi) though occasionally diamonds have
crystallized at depths about 300 kilometers (190 mi).
[11] 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.
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.
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.
Space diamonds
Primitive interstellar meteorites were found to contain carbon possibly in the form of diamond (Lewis et al. 1987).
[13] 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.
[14][15]
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.
[10] Popigai crater in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.
[16]
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.
[17] It was referred to as
Lucy, after the Beatles' song "Lucy in the Sky With Diamonds".
[18][19]
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
—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.
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.
[11]
The magma in volcanic pipes is usually one of two characteristic
types, which cool into igneous rock known as either kimberlite or
lamproite.
[11]
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
[11] 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.
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....
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