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Diamond, mineral form of the element carbon, valued as a precious stone. Diamond is the
hardest natural mineral and has many other exceptional properties that collectively make it an
important industrial and scientific material. Unique geologically, diamonds form at great depths
within Earth and are typically billions of years old.
II How Diamonds Form
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Diamonds are crystals composed of pure carbon. In nature, diamond crystallizes from hot
carbon-rich fluids. This crystallization requires tremendous heat and pressure—1000 to 1200°C
(1800 to 2200°F) of heat and 50 kilobars of pressure. (One bar is based on the pressure the
atmosphere exerts at sea level, about 1.02 kg per sq cm, or 14.7 lb per sq in; 50 kilobars is
50,000 bars.) The pressures and temperatures at which natural diamond forms only occur deep
underground. Scientists believe that diamonds form at depths greater than 150 km (93 mi), and
there is evidence that some diamonds formed as deep as 670 km (420 mi) beneath Earth’s
surface.
Concentrations of diamonds great enough to be economically feasible for mining are usually
found in Earth’s oldest continental regions, called cratons. Cratons form the cores of most
continents and consist of inactive geological areas more than 2 billion years old with thick crust
and deep roots extending into the mantle beneath. Craton conditions are ideal for diamond
formation and preservation. Scientists have determined the ages of some diamonds by dating
mineral impurities trapped within the diamonds. These data reveal that most cratonic diamonds
are ancient, some older than 3 billion years.
Much younger volcanic rocks—kimberlites and lamproites—pass through the cratonic rocks in a
liquid form called magma during their rapid ascent to Earth’s surface. These flowing veins of rock
act as carriers of diamonds and other rock fragments. After eruption they solidify, forming funnel-
shaped kimberlite “pipes.” These pipes are primary diamond deposits. Many diamonds are
recovered at a distance from their primary deposits in secondary alluvial deposits, which are
loose eroded materials left behind by flowing water. In some instances diamonds are also found
in sandstones, conglomerates, and other sedimentary rocks that presumably solidified from
former alluvial deposits. Wind and glaciers can also transport diamonds from their point of origin
at Earth’s surface.
Small, generally low quality diamonds form in rocks at shallower depths under pressure
conditions that are higher than usual for those depths. Tectonic movement, rather than magma,
transports these diamonds to Earth’s surface. Deposits of this type occur in areas such as
Kazakhstan and typically involve the collision of a continental and an oceanic plate followed by
rapid uplift of deeply buried rocks. Diamond deposits brought to the surface by tectonic
movement are generally younger than kimberlitic diamonds, and typically consist of
microdiamonds (less than 1 mm across) or graphite relics of larger diamonds.
Diamonds are also found in meteorites and near meteorite craters on Earth’s surface. Extremely
small diamonds (nanodiamonds) occur in many types of meteorites and have a lower density
than other diamonds. Meteorites can also produce pressure and heat at the moment of impact
sufficient to transform carbon into diamond. Diamond found in a type of meteorite called ureilite
is thought to form directly from graphite contained in the meteorites upon impact. Impact-crater
diamonds are opaque and range from very small to around a centimeter in diameter.
III Properties
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Diamond is the hardest natural substance known. This hardness is exhibited in diamond’s
resistance to scratching and its ability to scratch other materials. Steel and glass, for instance,
can be scratched by diamond. The Mohs hardness scale, devised by the German mineralogist
Friedrich Mohs to indicate relative hardness of substances on a rating scale from 1 to 10,
assigns diamond a value of 10. Diamond’s hardness is not a constant quantity but varies even
within a single diamond.
Diamonds are crystals composed of carbon atoms. Atoms in a crystal are arrayed in a regular
repeating pattern. A crystal’s outward form, bounded by smooth plane surfaces that meet at
predictable angles, reflects this internal order. Crystals tend to cleave, or split, along lines called
cleavage planes between layers of atoms. In the case of diamond crystals, each carbon atom is
bonded to four surrounding carbon atoms. This microscopic arrangement determines the
visible shape of diamond crystals, which are generally octahedrons (solid shapes with eight
faces). Individual diamond crystals therefore cleave cleanly along planes parallel to the faces of
an octahedron.
Two important properties, brilliance and fire, contribute to diamond’s beauty. Brilliance is the
fraction of the light that falls on a diamond that the diamond returns to the eyes of an observer—
the more light returned, the higher the brilliance. Diamond’s brilliance arises from its index of
refraction, which determines the angle at which light is bent as it crosses the boundary between
the air and the stone. Fire is the ability of a substance to split white light into rainbow colors—the
greater the separation between colors, the greater the fire. Diamond’s fire originates with its
dispersion, which is the difference in diamond’s index of refraction for light of different colors.
Diamond has both a higher index of refraction and a higher dispersion value than any other
natural, transparent, colorless material.
Diamonds exhibit a wide range of transparency and color. Transparency is a measure of the
amount of light that passes through a diamond rather than being absorbed. Colorless
diamonds, known as white diamonds, are most familiar, but green, blue, red, orange, yellow,
and brown diamonds also are known. Structural imperfections or dislocations and the presence
of trace elements, mainly nitrogen, cause color in diamonds. Some diamonds luminesce (emit
light) when exposed to sunlight or other ultraviolet-light sources. The light the diamonds emit is
usually light blue, but yellow, orange, and red luminescence occurs in some stones.
Most diamonds used as gems are single crystals large enough to be easily visible to the eye.
Diamond also occurs, however, in polycrystalline forms commonly known as ballas, bort, and
carbonado. Ballas is a compact, spherical mass of tiny diamond crystals of great hardness and
toughness. Bort is an extremely hard, dark, imperfectly crystallized diamond. The term bort
sometimes is also applied to minute fragments of gem diamonds. Carbonado is an opaque
grayish or black form of diamond that consists of microscopic crystals and has no cleavage.
Ballas, bort, and carbonado are all used industrially, in lapidary (gem-cutting) work, and as a
tough coating for the tips of drills and the edges of cutting tools.
Other characteristics of diamonds are frequently useful in identifying the stones and in
differentiating between true diamonds and imitations. Because diamonds are excellent
conductors of heat, they are cold to the touch and are sometimes called “ice.” Most diamonds do
not conduct electricity well, but diamonds do become charged with positive static electricity when
rubbed. Diamond resists attack by acids or bases. Since diamonds are a form of carbon, like
coal, they will burn, but only when heated to extremely high temperatures.
The density of diamond ranges between 3.15 and 3.53 g/cm3, but the density of pure diamond
is always very close to 3.52 g/cm3. Diamond is much denser than crystals composed of
elements of similar weight to carbon atoms because the carbon atoms in diamond are packed
tightly together. Quartz, for example, is composed of atoms of silicon and oxygen, both of which
are heavier than carbon atoms. The density of quartz, however, is only 2.65 g/cm3.
IV Diamond Cutting
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Rough diamonds are not brilliant and can appear greasy. Diamond cutting encompasses a
number of processes that bring out the beauty of gem diamonds. These processes include
cleaving, sawing or laser cutting, and polishing. A diamond cutter seeks to enhance the
brilliance and fire of each stone and to eliminate imperfections, such as cracks and cloudiness.
The cutter develops a plan that will accomplish these goals while still producing a gem of the
greatest size and hence maximum value. About half of a natural diamond’s size is lost in
diamond cutting.
Examining the stone is the first step in diamond cutting. The cutter determines where cleavage
planes lie and decides how the stone can best be divided by cleaving and sawing. Ink marks on
the rough diamond serve as a guide for the shaping to follow.
The cutter next places the diamond firmly in a holder for cleaving. A light blow of a hammer on
the cleaving iron, which is held against the diamond parallel to the cleavage plane, cleaves the
stone. In present-day practice cutters more often saw diamonds or cut them with a laser rather
than cleave them. The saw is a thin metal disk, the edge of which is impregnated with a mixture
of diamond dust and oil.
Polishing, the final step in the cutting of a diamond, consists of forming the facets of the finished
stone. Cutters most often choose the “brilliant” form, which has 58 facets. During the polishing
process a mount called a dop firmly holds the gem. A flat, horizontally revolving cast-iron wheel
coated with a mixture of diamond dust and oil forms the facets. The cutter holds the stone in its
dop against the surface of the wheel until the facet forms. In the course of polishing, the cutter
moves the stone many times in its dop to present new surfaces for polishing.
V Judging A Diamond’s Quality
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Only high-quality diamonds are suitable for use as gems. In judging the quality (and therefore
the value) of a cut diamond, a buyer must take into account four criteria, known as the “four C’s”:
color, clarity, carat weight, and cut. Colorless stones are extremely valuable, while yellow or
brown-tinged stones are regarded as imperfect. Fancy, colored diamonds, or fancies, exhibit
clear, strong colors such as blue, green, red, and orange. Fancies are quite rare and highly
prized. The presence or absence of internal blemishes and flaws determines clarity. Weight
reflects a diamond’s size. The unit of weight usually employed for diamonds and other gems is
the metric carat, which is equal to 0.2 g (about 0.007 oz). Another unit used to express the weight
of diamonds is the point, equal to 0.01 carat. A stone of 82 points would therefore weigh 0.82
carat. A 5-carat stone is worth more than five 1-carat stones that are otherwise of the same
quality. The final criteria buyers use in determining the quality of a diamond is its cut. The cut is
the shape and proportion of the stone, as determined during the diamond-cutting procedure.
VI Industrial Uses
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Eighty percent of natural diamonds produced from mining contain flaws with regard to color,
shape, or size or have impurities that make them unsuitable for use as gems. Ballas, bort, and
carbonado are examples of flawed diamonds. Such diamonds are classed as industrial grade
and used in manufacturing.
Due to its hardness, diamond, as either single crystals or finely ground powder, can be used in
cutting, abrading, or polishing tools. It is used extensively in drill bits, in cutting devices, and as a
surface that can withstand shock and pressure. Industrial diamond has applications in the
mining, oil and gas, electronic, optical, thermal, machining, dentistry, and medical industries.
Scientific applications include radiation- and photo-detection and experiments involving high
pressures and temperatures.
VII Synthetic And Imitation Diamond
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High demand for diamonds has led to the development of methods for producing artificial
diamonds. Artificial diamonds used in industry are generally known as synthetic diamonds;
artificial diamonds used for ornamentation are called imitation diamonds. Even though the
majority of natural diamonds are industrial grade, only about 10 percent of the diamonds used
for industrial purposes are natural diamonds. The other 90 percent are synthetic. The two most
common processes of synthesizing diamond are the high-temperature high-pressure (HTHP)
and chemical vapor deposition (CVD) methods. The HTHP method converts carbon to diamond
at high temperature and pressure using a molten metal catalyst. The HTHP method is
sometimes also used to change or enhance the colors of some rare natural diamonds, thus
making them more valuable on the market. The CVD method produces diamond coatings by
heating a hydrocarbon gas over a metal surface. These diamond coatings greatly extend the
lifetimes of precision dies, drills, and saw blades.
The high price of gem diamonds has created a market for imitation diamonds made from less
expensive materials. Minerals that strongly disperse light, including cubic zirconia, transparent
quartz (rock crystal), synthetic rutile, corundum, spinel, and moissanite (silicon carbide), are all
used as imitation diamonds. Other imitations are made from a lead glass known as paste or
strass. Devices called thermal conductivity probes can detect imitation diamonds because
imitation diamonds generally do not conduct heat as well as real diamonds. Imitation diamonds
can also be identified optically and are easily scratched by real diamond.
VIII History
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The word diamond is derived from the ancient Greek term adamas meaning “invincible,” which
the ancient Greeks probably applied to any hard stone. Sanskrit texts from India contain one of
the earliest references to diamonds and suggest that diamonds were known and ascribed
value at some time before 400 bc.
Roman literature of the 1st century ad contains another early reference to diamond, and the
diamonds known to the Romans undoubtedly came from India. India was for centuries the
greatest source of diamonds in the world and is known for most of the famous historical
diamonds. Although diamonds discovered in Borneo around 700 ad were an important source
for Southeast Asia, India was the only source of diamonds known to Europeans until the 18th
century. Europeans believed that diamonds were found only in the fabled mines of Golconda,
near present-day Hyderâbâd. Golconda was in fact the market city of the diamond trade, and
gems sold there came from a number of mines.
The earliest evidence of diamond cutting dates to the 1330s in Venice. Europeans began to
trade diamonds on a regular basis early in the 15th century with the opening of trade routes to
the east. A second source for Europeans became available when diamonds were discovered in
Brazil in 1726. The Brazilian diamond trade is the world’s main producer of ballas and
carbonado.
The identification of a “pebble” picked up by a child on the banks of the Orange River in South
Africa in 1866 as a 21-carat diamond precipitated the opening of diamond fields in that region.
The rush to search for alluvial diamonds in the gravel of the Orange and Vaal rivers greatly
accelerated in 1870 and 1871 following the discovery of “dry diggings” in the district near
present-day Kimberley. These diggings were roughly circular patches of yellow clay containing
diamonds. As the miners dug deeper in the clay, often called “yellow ground,” they found a hard,
bluish rock below it that also proved to be productive. This “blue ground,” a rock called
kimberlite, is the parent material from which yellow ground is formed by weathering. Further
mining disclosed that the circular areas of yellow and blue ground are the tops of funnel-shaped
volcanic pipes of kimberlite. Prospectors have found similar pipes, not all of which contain
diamonds, at various other locations in Africa and other continents.
South Africa quickly became the leading producer of diamonds in the world. During the late 19th
century De Beers Consolidated Mines Ltd., led by British colonial statesman Cecil Rhodes, was
formed there. To this day the De Beers cartel controls a large portion of the world’s natural
diamond production.
South Africa is still a leading producer of diamonds, but prospectors have found substantial
diamond deposits, many of which are alluvial, in other parts of Africa, including Botswana, the
Democratic Republic of the Congo (DRC, formerly Zaire), Tanzania, Ghana, and Sierra Leone.
Other major present-day diamond producers are Australia, Siberia in Russia, Brazil, Venezuela,
and Guyana. Canada may become a large diamond producer in the near future due to the
discovery of diamond deposits in the Northwest Territories and Alberta during the mid- and late
1990s.
Smaller natural diamond deposits occur in China, Borneo, Myanmar, Thailand, and the Ural
Mountains in Russia. Isolated stones have been found at various places in the United States,
and a kimberlite pipe on the Colorado-Wyoming state line yields sufficient quantities for mining.
Kimberlite diamonds also occur in Arkansas.
General Electric produced the first small synthetic diamonds of similar quality to natural
industrial diamonds in 1955. Today, industrial-grade synthetic diamonds are inexpensively
mass-produced.
IX Famous Diamonds
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A number of individual diamonds have become famous, primarily for their great size but also for
their exceptional color, cut, uniqueness, or history. The Great Mogul diamond, reputed to have
weighed 240 carats when cut, has disappeared since it was described by the French traveler
Jean Baptiste Tavernier in India in 1665. Some authorities believe that the Koh-i-noor diamond,
which now weighs 106.1 carats and is one of the British crown jewels, was part of the Great
Mogul. Jean Baptiste Tavernier is also associated with the Hope diamond, a 45.52-carat blue
diamond that originally weighed 110.5 carats. The Hope diamond is a recut version of the Great
Blue diamond. The Great Blue diamond was once part of the French crown jewels.
The Cullinan, the largest rough diamond ever found, was discovered in the Premier Mine in
South Africa in 1905. The government of the Transvaal, a British crown colony that later became
part of the Union of South Africa, presented the Cullinan to King Edward VII. The Cullinan
weighed 3,106 carats (1.37 lb) before cutting and was pronounced by crystallographers to be a
cleavage fragment of a considerably larger stone. When the stone was cut a total of 105 gems
were produced weighing 1,063 carats in all. The largest of these was a 530.2-carat drop-
shaped stone called the Star of Africa or Cullinan I. The Star of Africa is the largest cut diamond
in existence and is set in the British royal scepter.
The Vargas diamond, found in Brazil in 1938, weighed 726.6 carats in its uncut state. When cut
in 1945, it yielded 29 stones with a total weight of 411 carats. In 1934 a diamond of almost
precisely the same weight, the Jonker diamond, was discovered in an alluvial deposit near the
Premier Mine. The Jonker is the finest-quality large diamond ever found. It was cut into 12 gems
ranging from 125.4 to 5.3 carats in weight. In 1967 the Lesotho diamond was discovered, also
in South Africa. It weighed 601.3 carats uncut. Other famous diamonds include the Regent, the
Sancy, the Tiffany, the Orlov, and the Dresden Green.
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