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, the French scientist 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, the English
chemist 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 uses of diamonds today are as gemstones used for adornment, and as industrial
abrasives for cutting hard materials. The dispersion of white light into spectral colors is the primary
gemological characteristic of gem diamonds. In the 20th century, experts in gemology 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 (its weight), cut (quality of the cut is graded
according to proportions, symmetry and polish), color (how close to white or colorless; for fancy
diamonds how intense is its hue), and clarity (how free is it from inclusions).[11] A large, flawless
diamond is known as a paragon.

Geology
Diamonds are extremely rare, with concentrations of at most parts per billion in source
rock.[12] Before the 20th century, most diamonds were found in alluvial deposits. Loose diamonds are
also found along existing and ancient shorelines, where they tend to accumulate because of their
size and density.[13]:149 Rarely, they have been found in glacial till (notably in Wisconsin and Indiana),
but these deposits are not of commercial quality.[13]:19 These types of deposit were derived from
localized igneous intrusions through weathering and transport by wind or water.[14]
Most diamonds come from the Earth's mantle, and most of this section discusses those diamonds.
However, there are other sources. Some blocks of the crust, or terranes, have been buried deep
enough as the crust thickened so they experienced ultra-high-pressure metamorphism. These have
evenly distributed microdiamondsthat show no sign of transport by magma. In addition, when
meteorites strike the ground, the shock wave can produce high enough temperatures and pressures
for microdiamonds and nanodiamonds to form.[14] Impact-type microdiamonds can be used as an
indicator of ancient impact craters.[15] Popigai crater in Russia may have the world's largest diamond
deposit, estimated at trillions of carats, and formed by an asteroid impact.[16]
A common misconception is that diamonds are formed from highly compressed coal. Coal is formed
from buried prehistoric plants, and most diamonds that have been dated are far older than the
first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds
formed in this way are rare, and the carbon source is more likely carbonate rocks and organic
carbon in sediments, rather than coal.[17][18]

Surface distribution
Geologic provinces of the world. The pink and orange areas are shieldsand platforms, which together constitute
cratons.

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford's rule
states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores
of continents with typical ages of 2.5 billion years or more.[14][19]:314 However, there are exceptions.
The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is
located in a mobile belt, also known as an orogenic belt,[20] a weaker zone surrounding the central
craton that has undergone compressional tectonics. Instead of kimberlite, the host rock is lamproite.
Lamproites with diamonds that are not economically viable are also found in the United States, India
and Australia.[14] In addition, diamonds in the Wawa beltof the Superior province in Canada and
microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre.[14]
Kimberlites can be found in narrow (1–4 meters) dikes and sills, and in pipes with diameters that
range from about 75 meters to 1.5 kilometers. Fresh rock is dark bluish green to greenish gray, but
after exposure rapidly turns brown and crumbles.[21] It is hybrid rock with a chaotic mixture of small
minerals and rock fragments (clasts) up to the size of watermelons. They are a mixture
of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces
of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the
eruption. The texture varies with depth. The composition forms a continuum with carbonatites, but
the latter have too much oxygen for carbon to exist in a pure form. Instead, it is locked up in the
mineral calcite (CaCO3).[14]
All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals
(melilite and kalsilite) that are incompatible with diamond formation. In kimberlite, olivine is large and
conspicuous, while lamproite has Ti-phlogopite and lamprophyre has biotite and amphibole. They
are all derived from magma types that erupt rapidly from small amounts of melt, are rich
in volatiles and magnesium oxide, and are less oxidizing than more common mantle melts such
as basalt. These characteristics allow the melts to carry diamonds to the surface before they
dissolve.[14]

Exploration
Diavik Mine, on an island in Lac de Gras in northern Canada.

Kimberlite pipes can be difficult to find. They weather quickly (within a few years after exposure) and
tend to have lower topographic relief than surrounding rock. If they are visible in outcrops, the
diamonds are never visible because they are so rare. In any case, kimberlites are often covered with
vegetation, sediments, soils or lakes. In modern searches, geophysical methods such
as aeromagnetic surveys, electrical resistivity and gravimetry, help identify promising regions to
explore. This is aided by isotopic dating and modeling of the geological history. Then surveyors must
go to the area and collect samples, looking for kimberlite fragments or indicator minerals. The latter
have compositions that reflect the conditions where diamonds form, such as extreme melt depletion
or high pressures in eclogites. However, indicator minerals can be misleading; a better approach
is geothermobarometry, where the compositions of minerals are analyzed as if they were in
equilibrium with mantle minerals.[14]
Finding kimberlites requires persistence, and only a small fraction contain diamonds that are
commercially viable. The only major discoveries since about 1980 have been in Canada. Since
existing mines have lifetimes of as little as 25 years, there could be a shortage of new diamonds in
the future.[14]

Ages
Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. Depending on
the elemental abundances, one can look at the decay of rubidium to strontium, samarium to
neodymium, uranium to lead, argon-40 to argon-39, or rhenium to osmium. Those found in
kimberlites have ages ranging from 1 to 3.5 billion years, and there can be multiple ages in the same
kimberlite, indicating multiple episodes of diamond formation. The kimberlites themselves are much
younger. Most of them have ages between tens of millions and 300 million years old, although there
are some older exceptions (Argyle, Premierand Wawa). Thus, the kimberlites formed independently
of the diamonds and served only to transport them to the surface.[12][14] Kimberlites are also much
younger than the cratons they have erupted through. The reason for the lack of older kimberlites is
unknown, but it suggests there was some change in mantle chemistry or tectonics. No kimberlite has
erupted in human history.[14]

Origin in mantle

Eclogite with centimeter-size garnet crystals.

Most gem-quality diamonds come from depths of 150 to 250 kilometers in the lithosphere. Such
depths occur below cratons in mantle keels, the thickest part of the lithosphere. These regions have
high enough pressure and temperature to allow diamonds to form and they are not convecting, so
diamonds can be stored for billions of years until a kimberlite eruption samples them.[14]
Host rocks in a mantle keel include harzburgite and lherzolite, two type of peridotite. The most
dominant rock type in the upper mantle, peridotite is an igneous rock consisting mostly of the
minerals olivine and pyroxene; it is low in silica and high in magnesium. However, diamonds in
peridotite rarely survive the trip to the surface.[14] Another common source that does keep diamonds
intact is eclogite, a metamorphic rock that typically forms from basalt as an oceanic plate plunges
into the mantle at a subduction zone.[12]
A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660
kilometers, a region that includes the transition zone. They formed in eclogite but are distinguished
from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A
similar proportion of diamonds comes from the lower mantle at depths between 660 and 800
kilometers.[12]
Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition
from graphite occurring at greater temperatures as the pressure increases. Thus, underneath
continents it becomes stable at temperatures of 950 degrees Celsius and pressures of 4.5
gigapascals, corresponding to depths of 150 kilometers or greater. In subduction zones, which are
colder, it becomes stable at temperatures of 800 degrees C and pressures of 3.5 gigapascals. At
depths greater than 240 km, iron-nickel metal phases are present and carbon is likely to be either
dissolved in them or in the form of carbides. Thus, the deeper origin of some diamonds may reflect
unusual growth environments.[12][14]

Carbon sources
The amount of carbon in the mantle is not well constrained, but its concentration is estimated at 0.5
to 1 parts per thousand.[14] It has two stable isotopes, 12C and 13C, in a ratio of approximately 99:1 by
mass. This ratio has a wide range in meteorites, which implies that it was probably also broad in the
early Earth. It can also be altered by surface processes like photosynthesis. The fraction is generally
compared to a standard sample using a ratio δ13C expressed in parts per thousand. Common rocks
from the mantle such as basalts, carbonatites and kimberlites have ratios between -8 and -2. On the
surface, organic sediments have an average of -25 while carbonates have an average of 0.[12]
Populations of diamonds from different sources have distributions of δ13C that vary markedly.
Peridotitic diamonds are mostly within the typical mantle range; eclogitic diamonds have values from
-40 to +3, although the peak of the distribution is in the mantle range. This variability implies that
they are not formed from carbon that is primordial (having resided in the mantle since the Earth
formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds)
not necessarily the same tectonic processes that act in the present.[14]

Formation and growth

Diamonds in the mantle form through a metasomatic process where a C-O-H-N-S fluid or melt
dissolves minerals in a rock and replaces them with new minerals. (The vague term C-O-H-N-S is
commonly used because the exact composition is not known.) Diamonds form from this fluid either
by reduction of oxidized carbon (e.g., CO2or CO3) or oxidation of a reduced phase such
as methane.[12]
Using probes such as polarized light, photoluminescence and cathodoluminescence, a series of
growth zones can be identified in diamonds. The characteristic pattern in diamonds from the
lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence
and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds
from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher
temperatures and pressures as well as the transport of the diamonds by convection.[14]

Transport to the surface

Diagram of a volcanic pipe

Geological evidence supports a model in which kimberlite magma rose at 4–20 meters per second,
creating an upward path by hydraulic fracturing of the rock. As the pressure decreases, a vapor
phase exsolves from the magma, and this helps to keep the magma fluid. At the surface, the initial
eruption explodes out through fissures at high speeds (over 200 meters per second). Then, at lower
pressures, the rock is eroded, forming a pipe and producing fragmented rock (breccia). As the
eruption wanes, there is pyroclastic phase and then metamorphism and hydration
produces serpentinites.[14]

In space
Main article: Extraterrestrial diamonds
Although diamonds on Earth are rare, they are very common in space. In meteorites, about 3
percent of the carbon is in the form of nanodiamonds, having diameters of a few nanometers.
Sufficiently small diamonds can form in the cold of space because their lower surface energy makes
them more stable than graphite. The isotopic signatures of some nanodiamonds indicate they were
formed outside the Solar System in stars.[22]
High pressure experiments predict that large quantities of diamonds condense from methane into a
"diamond rain" on the ice giant planets Uranus and Neptune.[23][24][25] Some extrasolar planets may be
almost entirely composed of diamond.[26]
Diamonds may exist in carbon-rich stars, particularly white dwarfs. One theory for the origin
of carbonado, the toughest form of diamond, is that it originated in a white dwarf
or supernova.[27][28] Diamonds formed in stars may have been the first minerals.[29]

Material properties