DEFINITION OF DIAMOND

Stable crystalline forms of pure carbon that crystallize

as isometric tetrahedral crystals. Diamonds form only

at high pressures in cool locations in the Earth’s mantle and

thus are restricted in their genesis to places in the subcontinental

mantle where these conditions exist, at 90–125 miles

(150–200 km) depth. Most diamonds that make their way

back to the surface are brought up from these great depths by

rare explosive volcanic eruptions known as kimberlites. More

rarely small diamonds, often as inclusions in other minerals,

are exposed along major thrust faults that have exposed

rocks from the mantle.

Diamonds are the hardest substance known and are

widely used as a gemstone. Uncut varieties may show many

different crystal shapes and many show striated crystal faces.

They have concoidal fracture, have a greasy luster, and may

be clear, yellow, red, orange, green, blue, brown, or even

black. Triangular depressions are common on some crystals,

and others may form elongate or even pear-shaped forms.

Diamonds have been found in alluvial deposits such as gravel,

and some mines have been located by tracing the source of the

gravel back to the kimberlite where the diamonds were

brought to the surface. Some diamond-mining operations such

as those of the Vaal River, South Africa (discovered in 1867),

proceeded for many years before it was recognized that the

source was in nearby kimberlites.

Dating the age of formation of small mineral inclusions

in diamonds has yielded some very important results. All

dated diamonds from the mantle appear to be Precambrian in

age, with one type being up to 3.2 billion years old, and

another type being 1.0–1.6 billion years old. Since diamonds

form at high pressures and low temperatures, their very existence

shows that the temperature deep in the Earth beneath

the continents in the Precambrian was not much hotter than

it is today. The diamonds were stored deep beneath the continents

for billions of years before being erupted in the kimberlite

pipes.

See also KIMBERLITE; PRECAMBRIAN.

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