Diamond is the king of gems. It is a form of pure carbon, and it is the
hardest substance known; only diamond will cut diamond. It is
interesting that the humble graphite, its close relative, is also pure
carbon, but graphite is so soft that it is used as a lubricant and for
making the “lead” in pencils.
The ancients believed diamond to be indestructible, and even today many
people believe that diamond cannot be broken. Despite its great
hardness, however, diamond is not exceptionally tough, and it can be
split along what diamond cutters call its _grain_.
The diamond’s high brilliance results from its very high refraction, or
ability to bend light, and its fire is caused by its high dispersion, or
ability to divide light into its rainbow colors. However, only in
properly cut stones are diamond’s brilliance and fire developed to their
maximum.
At great depths in the crust of the earth and under conditions of very
high pressure and temperature, diamonds form in pipe-like bodies of
kimberlite, a heavy dark rock consisting primarily of two minerals,
pyroxene and olivine. In South Africa diamonds are mined from the
kimberlite, but they also are recovered there and elsewhere from beds of
sand and gravel where they have accumulated after being released from
their mother rock by erosion.
The world’s largest diamond deposits are in Africa, and names such as
Congo, Sierra Leone, and the Union of South Africa bring to mind
colorful legends of fabulous discoveries of diamond. Smaller deposits
are found in South America—in Brazil, British Guiana, and Venezuela—and
in Asia. Even in the United States some diamonds have been found.
India was the most important source of diamond until 1728, when
discoveries were made in Brazil. Among the important large diamonds
found in India were the Koh-i-noor, the Great Mogul, and, very likely,
the Hope Diamond. Like India, Brazil in turn declined as a major source
of diamond with the discovery and efficient recovery of large quantities
in South Africa.
[Illustration: The Hope Diamond, because of its long and dramatic
history and its rare deep-blue color, is probably the best known
diamond in the world. By speculation, the Hope is linked to the
famous “French Blue,” which was brought to France from India in 1668
to become part of the crown jewels of Louis XIV. The French Blue was
stolen in 1792 and never recovered, but in 1830 an extraordinary
44.5-carat blue diamond—presumably cut from the missing gem—came on
the market. It was purchased by Henry Thomas Hope of England and
became known by its present name. In 1949 the gem was acquired from
the estate of Mrs. Evalyn Walsh McLean by Harry Winston Inc., of New
York. Ten years later, Harry Winston, Inc., presented the gem (shown
here in actual size) to the Smithsonian Institution.]
Diamonds are extremely rare even in diamond mines. For example, the
famous South African mines contain only one part of diamond in more than
14 million parts of worthless rock. In spite of this, more than three
tons of gem- and industrial-quality diamond were mined in 1963.
Among the British crown jewels is a cut diamond weighing 530.20 carats
(more than 3¾ ounces), one of several stones that were cut from the
largest gem diamond ever discovered. The rough stone, known as the
Cullinan Diamond, weighed 3106 carats (almost 1¾ pounds) when it was
found at the Premier Mine in South Africa in 1905.
[Illustration: The Portuguese Diamond, weighing 127 carats, is the
13th largest cut diamond on record. More unusual, it is from Brazil,
and is thought to have been part of the Portuguese crown jewels. In
addition to its brilliant color flashes, it has a slight milky
fluorescence that causes it to “glow” even in artificial light.
(Actual size.)]
Diamonds vary from colorless to black and from transparent to opaque. As
they come from the mines, they are graded into two groups, gem and
industrial. Those whose color, imperfection, or shape make them useless
as gems—more than 8 out of every 10 carats mined—are used in industry.
Diamonds of industrial quality also are produced synthetically, and
these are used primarily in the manufacture of grinding wheels.
The best gem diamonds are flawless and are colorless or slightly blue.
Their value depends on their color, clarity, cut, and carat weight. Most
costly are those called fancies, which have a distinct color such as
blue, pink, green, or deep yellow.