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In mineralogy, diamond (from the ancient Greek αδάμας – adámas
"unbreakable") is an 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 ambient 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.
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, purple, pink, orange or red. Diamond also has
relatively high optical dispersion, that is ability to disperse light of
different colors, which results in its characteristic luster. Excellent
optical and mechanical properties, combined with efficient marketing,
make diamond the most popular gemstone.
Most natural diamonds are formed at high-pressure high-temperature
conditions existing at depths of 140 to 190 kilometers (87 to 120 mi) in
the Earth 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 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
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 specially developed to distinguish
natural and synthetic diamonds and diamond simulants.
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