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subject: Diamonds- The Gemstone Series [print this page]

The diamond is the most important of the gemstones formed when pure carbon

is subjected to high pressures and temperatures of the Earths deep inte-rior.

Microdiamonds have also been produced by meteorite impacts, whereby

the high-pressure collisions fused carbon into diamond dust. Diamonds with

small black inclusions of foreign minerals are used as evidence that continental

material can get trapped in subduction zones and pulled deep down into

the mantle, where high pressures compress carbon atoms into tight crystals.

Diamonds also can be produced synthetically in the laboratory by compressing

carbon to extreme temperatures and pressures.

Diamond crystals are usually six-sided or eight-sided, although they

are often found in distorted and irregular shapes. A diamonds value

depends on its hardness and its brilliance, derived from a high index of

refraction, or bending of light rays, which accounts for its so-called fire

produced by cutting the stone in such a way that light is refracted and highly

dispersed as it passes through the gemstone.The value of a cut diamond

depends on its color, purity, and size, as well as the skill with which it is cut.

In general, the most valuable diamonds are those flawless stones that are

colorless or possess a blue-white color. However, the term blue-white diamond

is sometimes misused by jewelers to describe stones of inferior quality.

A faint straw-yellow color that diamonds often possess detracts from the

gems value.

If diamonds are colored deep shades of yellow, red, green, or blue, they

are called fancy stones and are greatly prized and command very high prices.

Diamonds can be colored deep shades of green by bombardment with nuclear

radiation or blue by exposure to high-energy electrons.A stone colored green

by irradiation can be heated to bring out a deep yellow hue.These artificially

colored stones are difficult to distinguish from natural ones.

Diamonds have been discovered in many localities throughout the

world, but only in a few places are they plentiful enough to be mined commercially.

Most commonly, diamonds are found in alluvial or placer

deposits derived from eroded volcanic mountains.They accumulate in these

deposits because of their inert chemical nature, great hardness, and fairly

high density. The earliest diamonds were mined from stream gravels in the

southern and central portions of India. An estimated 12 million carats (the

carat is a unit of weight for gemstones equal to 0.2 gram) was produced

from Indian mines. India was virtually the only source of diamonds until

they were discovered in Brazil in 1725. East central Brazil has produced

about 160,000 carats annually, chiefly from stream gravels near the city of

Diamantina, Minas Gerais.

Today, about 95 percent of the worlds output of diamonds is from

Africa.The Congo is by far the largest producer, supplying more than 50 percent

of the global demand.The diamonds are mostly industrial grade, used for

cutting tools, and represent only a fraction of the worlds total value of diamonds

produced. Industrial diamonds are also produced synthetically by subjecting

pure carbon to extreme temperatures and pressures similar to those

found deep inside the Earth. Several million carats are manufactured yearly,

but these diamonds are not suitable for cutting into gemstones because of

their small size.

Although some gem quality diamonds are still recovered from gravels,

the principal South African production is from kimberlite pipes,

named for the town of Kimberley, South Africa. They are composed of

jumbled fragments of mantle rocks, which are believed to have originated

as deep as 150 miles below the surface.The intrusive bodies vary in size and

shape, although many are roughly circular and pipe-shaped. Prospecting in

South Africa for diamonds has uncovered over 700 kimberlite pipes and

other intrusive bodies. However, most of these were found to be barren of

diamonds.

The kimberlite deposits were originally worked as open pits, but as the

mines became deeper, underground mining methods had to be employed. At

the Kimberley mine, the worlds deepest diamond mine, the diameter of the

pipe at the surface was about 1,000 feet, and the width decreased sharply with

depth. Mining stopped in 1908 at a depth of 3,500 feet because of flooding,

even though the diamond-rich pipe continued on to greater depths. At the

surface of the mine, the kimberlite is weathered to a soft yellow rock, and at

depth the rock grades into a harder blue rock.The ratio of diamonds to barren

rock was about 1 to 8 million by weight.The diamonds are extracted from

the blue rock by first crushing it finely enough to permit concentration. It is

then spread out on tables coated with grease, to which the diamonds adhere

while the waste material is washed away.

The worlds largest and most productive diamond mine is the Premier

mine, located 24 miles east of Pretoria, South Africa. Since mining began in

1903, more than 30 million carats, or about six tons of diamonds, have been

produced. The worlds largest diamond, the Cullinan, weighing 3,024 carats

(21 ounces), was found there in 1905.

Diamonds have been found sparingly in other regions of the world,

including Guyana,Venezuela,Australia, and various parts of the United States.

Small stones have occasionally been discovered in stream sands along the eastern

slope of the Appalachian Mountains from Virginia to Georgia. Diamonds

from the gold sands of northern California and southern Oregon have been

reported. Sporadic occurrences have also been found along the border of

Wyoming and Colorado and in glacial till deposits in Ohio,Wisconsin, and

Michigan.

In 1906, diamonds were discovered in a kimberlite pipe near

Murfreesboro, Arkansas. This locality resembled areas containing diamond

pipes in South Africa and was the site of the only operating diamond mine in

the United States, yielding a total of about 40,000 stones. It is now

a tourist attraction, known as the Crater of Diamonds State Park, where people

pay a small fee to sift through the black soil in search of instant wealth.

The diamond field is plowed regularly, and generally the period after a rainstorm

is the best time to search for diamonds because the surface of the freshly

turned soil is washed, exposing the diamonds, which might otherwise look

like bits of glass. One of the largest diamonds was found by a baby sucking on

what was thought to be an ordinary rock.

Joseph Kieffer

http://hand-made-jewelry.com

by: joserj3gki

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