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Diamond is one of the two best
known forms (or allotropes) of carbon, whose hardness and high dispersion of
light make it useful for 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 only be
scratched by other diamonds, 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.
The name "diamond" derives from the ancient Greek adamas (ad?µa?; "impossible to
tame"). They have been treasured as gems since their use as religious icons in
India at least 2,500 years ago.
Popularity of diamonds has risen since the 19th century because of improved
cutting and polishing techniques, and they are commonly judged by the "four Cs":
carat, clarity, color, and cut.
Most natural diamonds originate from central and southern Africa, although
significant sources of the mineral have been discovered in Canada, 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.
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