Diamonds are almost for ever

Press, 5 March 1987, Page 21

Diamonds are almost for ever

Diamond is the most prized of precious stones, and its flashing beauty and rarity have been coveted and valued for thousands of years as a symbol of wealth and power.

It is the hardest known mineral, natural or artificial, and this is reflected in the origin of its name from the Greek “Adamas,” meaning “unconquerable.”

In the natural state, a gem diamond may look like an unattractive, rounded lump of glass. Its flawless transparency and brilliant lustre do not become apparent until it has been meticulously cut and polished. Even so, not all diamonds are of gem quality; most are used in a crushed or powdered form as cutting and polishing agents in the engineering industry. Initially, India had the monopoly on diamonds, and an Englishman visiting the main source, Golconda, near Hyderabad, in 1622, noted that 30,000 men were at work in the mines. The monopoly was broken in 1725 when diamonds were discovered in Brazil.

In 1866, the first alluvial diamonds were found in South Africa, followed by the discovery of very rich “fields” on three neighbouring farms. A mining camp grew on these farms and, in 1873, became known as

Kimberley. Not only were the stones found on the surface, but also below it in a rock called “blue-ground.” To mine the “blue-ground” a pit was dug, which by 1881 was 400 feet deep. The small-time prospector was eventually forced out by large mining companies such as De Beers, which developed systematic deep level exploitation on a grand scale. Since then, important sources of diamonds have been found in other parts of Africa, North America, the Soviet Union, and Australia.

Diamond is an enigma, for despite its hardness, its composition is carbon, exactly the same as that of a humble lump of graphite. The difference is that diamond forms from carbon under extremely high pressures, 4700 times that of the atmosphere, pressures which are found only very deep below the earth’s surface. This results in the carbon atoms having a very uniform and close-set arrangement. Graphite is carbon formed from peat at lesser pressures and its atoms are much less strongly

We now believe that the source of diamonds lies 200 km below the surface, within the mantle, the wide zone between the Earth’s core and its skin-like crust. Diamonds have been carried to the surface by being injected to higher levels in a liquid rock, which on cooling is called kimberlite, .or “blueground.” The injection forms carrotshaped pipes clustered along deep-seated fractures in continental crust. Kimberlite, named after the South African town where it was first found, contains many of the minerals thought to compose the mantle.

Diamonds are found in all of the major continental areas, but by far the greatest numbers have been derived from the African continent, which yields over 44

million carats of diamonds a year. Nearly 35 million carats of these are obtained from kimberlite pipes but are largely of industrial grade. Gem quality diamonds are usually found in alluvial deposit which, although producing less than a third of the total yield, contain the most valuable stones.

Following their emplacement millions of years ago, kimberlite pipes have been affected by the erosive action of rivers and the sea for many eras. Being almost indestructible, diamonds loosened from the kimberlite or “blue-ground” have been washed down rivers to become incorporated, almost unscathed, within ancient gravels and shell beds millions of years old, as well as in more recent river and coastal deposits. In many parts of the world,

such as Australia and the mouth of the Orange River in Africa, it is these deposits that are eagerly sought and mined, because the weaker, flawed diamonds have disintegrated “along the wayside” and only the better quality gem diamonds have survived. A case in point concerns a bed of fossil oyster shells near the mouth of the Orange River. High quality diamonds washed from kimberlite pipes emplaced 90 million years before, more than 2000 kilometres upstream, had become incorporated into this shell bed in such quantities that when news of the discovery spread around the world in 1927 the Government had to intercede. It placed a cast slab of concrete over the shell bed to prevent the diamond market from being flooded. New Zealand has no diamond deposits of its own, but Canterbury Museum has a display in its geology hall that contains glittering replicas of some of the world’s most renowned cut diamonds.

Uniform and

The Cullinan Stone was found in South Africa in 1905 and was given to King Edward VII of England on his birthday in 1907 by the Transvaal Government. The stone was cut into 105 brilliant shapes in 1908, taking

three craftsmen 14 hours a day for eight months to complete. The replicas of four very rare coloured diamonds are also on exhibit in the museum, including the Dresden, a 41 carat bright green Indian stone; the yellow South African Tiffany, 125 carats, and the blue Indian Hope of 44 carats, reputed to bring ill-luck to all its possessors. There is reference in the display to a diamond rush in Canterbury which came to an abrupt end when the tiny clear crystals proved to be common quartz — but that is another story.

close-set

By

MARGARET A. BRADSHAW