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The mere mention of
the phrase diamonds are forever fills the mind with a multitude of
concepts and images. Diamond is a mineral, a natural crystalline
substance, the transparent form of pure carbon. The hardest surface
known to man. The diamond is exotic, formed in Earth's interior and
shot to the surface by extraordinary volcanoes. A diamond is likely
the oldest thing you will ever own, probably 3 billion years in age,
fully two-thirds the age of the Earth.
All diamonds are at
least 990,000,000 years old. Many are 3,200,000,000 years old (3.2
billion years). Diamonds are formed deep within the Earth: between
100 km and 200 km below the surface. Diamonds form under remarkable
conditions. The temperatures are about 900 - 1300 C in the part of
the Earth's mantle where diamonds form.
Diamonds are carried to the surface by volcanic eruptions. The
volcanic magma conduit is known as a kimberlite pipe or diamond
pipe. We find diamonds as inclusions in the (rather ordinary
looking) volcanic rock known as kimberlite. A diamond is made of
carbon, yet the stable form of carbon at the Earth's surface is
graphite. To ensure they are not converted to graphite, diamonds
must be
transported extremely rapidly to the Earth's surface. It is probable
that kimberlite lavas carrying diamonds erupt at between 10 and 30
km/hour. Within the last few kilometers, the eruption velocity
probably increases to several hundred km/hr or beyond the speed of
sound. This emphasizes that diamonds do not form in the kimberlite
magma, but are carried up to the surface by the magma.
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The earliest productive diamond mines were located in the Golconda
region of
India, especially
along the Kristna river. After 1725 this mining district was
eventually diminished in importance by the diamond deposits in
Brazil. Diamonds were first mined in
Brazil along the Jequitinhonha
river, area in the Diamantina city (Translated as Diamond Land) of
the state of Minas Gerais. Only twenty percent of mined diamonds are
of gem quality. Eighty percent of these are sold in a
'managed selling environment', and the remainder are used for
industrial purposes.
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