Diamond History Natural history 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, but at a
comparatively low temperature range between approximately 1652-2372 °F (900-1300 °C).These conditions are
known to be met in two places on Earth; in the lithospheric mantle below relatively stable continental plates,andat the site of a meteorite strike. 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) though occasionally diamonds have crystallized
at depths of 300-400 km (180-250 miles) as well.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.
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.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. 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. 
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. 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 was deposited there via an asteroid impact (not formed from the impact) about
3 billion years ago.These diamonds formed in the intrastellar environment. Presolar grains in many meteorites found on earth contain nanodiamonds of extraterrestrial origin, probably
formed in supernovas. White dwarf stars have been described as having a carbon core and were hyped in a 2004 news headline as diamond. Surfacing Diamond-bearing rock is brought close to the surface through deep-origin volcanic eruptions. The magma for such
a volcano must originate at a depth where diamonds can be formed,150 km (90 miles) deep 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/or 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. The magma in volcanic pipes is usually one of two characteristic types, which cool into igneous rock known as
either kimberlite or lamproite.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 which are often
altered to serpentine by heat and fluids during and after eruption. Certain indicator minerals typically
occur within diamondiferous kimberlites and are used as mineralogic 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 chromian garnets (usually bright red Cr-pyrope, and occasionally green ugrandite-series garnets),
eclogitic garnets, orange Ti-pyrope, red high-Cr spinels, dark chromite, bright green Cr-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, eroded out of their
kimberlite or lamproite matrix, accumulate 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
approximate size and density. Diamonds have also rarely been found in deposits left behind by glaciers
(notably in Wisconsin and Indiana); however, in contrast to alluvial deposits, glacial deposits are not
known to be of significant concentration and are therefore not viable commercial sources of diamond.  History and gemological characteristics Diamonds are thought to have been first recognized and mined in India (Golconda being one of them), where
significant alluvial deposits of the stone could then be found along the rivers Penner, Krishna and Godavari.
Diamonds have been known in India for at least 3000 years but most likely 6000 years.The most familiar
usage of diamonds today is as gemstones used for adornment a usage which dates back into antiquity. The
dispersion of white light into spectral colors, is the primary gemological characteristic of gem diamonds.
In the twentieth century, experts in the field of 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.
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