Diamond Fact Sheet

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Introduction

Diamond is composed of carbon and is the hardest known substance although it can be shattered by a sharp blow. Its high refractive index and power to disperse light gives diamond its brilliance and fire. If heated to bright red it will burn. The weight of diamond is measured in carats (one carat = 0.2 gram). A one carat, cut and polished diamond is about 6.5 millimetres across.

Diamonds were first discovered in India and Alexander the Great was the first to bring them from India to Europe in 327 BC. Alluvial gold miners in Brazil found what they described as 'curious pebbles' in 1725, which was subsequently identified as diamonds. Alluvial diamonds were discovered in 1859 in South Africa and in 1869 mining of the hard rock sources of these alluvial diamonds started. Today, diamonds are also mined in Botswana, Zaire, Australia and Russia.

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Occurrence

Diamond occurs naturally but is extremely rare compared to other minerals. Large quantities of industrial grade synthetic diamonds are made in several countries. It is possible also to make gem quality diamonds.

Diamond is thought to form 150-200km below the Earth's surface, where high temperature (1050-1200°C) and pressure (45-55 kilobars) allow it to crystallise. They are picked up by hot molten rock (magma) intruding up into the Earth's crust. These intrusions form narrow cylindrical bodies called 'pipes' and only a very small proportion have significant diamond content. When pipes are eroded, liberated diamonds may accumulate in alluvial deposits. Diamonds may be found far from their source as their hardness allows them to survive multiple episodes of erosion and deposition.

The quality of diamonds can be split broadly into gem, near gem, industrial and boart categories. In rare cases 90% of diamonds in a deposit are of gem quality; but most economic deposits contain 20 to 40% gem quality diamonds.

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Australian Resources and Deposits

Diamonds in Australia were recorded in the Bathurst area, New South Wales in 1851. Significant quantities also were mined from alluvial deposits at Copeton and Bingara, near Inverell in north-eastern New South Wales from 1867 to 1922 although minor production resumed at Copeton in 1997. Numerous minor occurrences of diamond have been recorded elsewhere in Australia.

Most of Australia's economic diamond resources are in the very large, low gem quality content Argyle deposit, in the Kimberley region of north-western Australia, accounting for about 90% of Australia's diamond production. Relatively small resources of alluvial diamonds are worked nearby in Smoke Creek (Argyle Alluvials). A number of small diamond 'pipes' with much higher gem quality diamond content than Argyle are mined at Merlin in the Northern Territory.

The Kimberley region was originally selected as likely to contain diamond deposits because of its similarity to the African diamond fields. Creek sediment sampling and other exploration techniques in the early to mid 1970s led to the discovery of 23 diamondiferous but low grade pipes at Ellendale in 1976 and the Argyle deposit in 1979. Shark cages were used in the late 1980s to protect scuba divers against crocodiles while they sampled diamonds (eroded from land) from the seabed off river mouths in the north-east Kimberley region. Small numbers of gem quality diamonds have since been recovered using an airlift drill attached to a boat.

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Australia in the World

The Argyle deposit contains a large proportion of the world's economic diamond resources. It has some of the highest diamond grades in the world and produces nearly twice the diamond output of any other country. Gem diamond production is probably comparable to many of the world's other mines but as only 5% is of gem quality (40% near gem, 55% industrial) the total value of production is less than that of several other countries.

About 70% of the world market for rough diamonds is controlled by the London-based Central Selling Organisation (CSO). The CSO was formed in 1934 in response to cyclical price slumps and the total closure of diamond mines in South Africa during the Great Depression. Argyle sells 100% of its product direct to the world market. The best Argyle rough diamonds, including the renowned pink diamonds, are cut and polished at the Argyle Diamonds facility in Perth, Western Australia.

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Mining and Processing

Large-scale, open-pit mining is used to extract diamond ore at Argyle. Broken ore is loaded onto 150 tonne trucks for transport to crushers where it is reduced to fragments measuring 18 millimetre or less. Any diamonds larger than 18 millimetres would be broken. Extraction of these large diamonds is uneconomic as statistical sampling indicates they are extremely uncommon and of poor quality. The crushed ore is washed in large rotating drums called scrubbers and fed to gently sloped, vibrating, double decked, slotted screens which sieve out three size fractions.

Finely ground ferrosilicon is mixed with water to make a thick, dense liquid (heavy media). When added to this liquid the lighter unwanted rock of the screened ore is more buoyant than the heavier diamonds. This buoyancy difference is used to separate the diamonds by pumping the liquid and its contents into downward-angled, metal cones (cyclones) in a descending spiral flow. The unwanted light material exits the top of the cyclone and diamonds and other heavy minerals exit at the bottom.

At the diamond recovery plant the heavy minerals pass through radial X-ray sorters, developed to cope with the unusually high diamond content of the Argyle ore. Diamonds, unlike most of the other heavy minerals, fluoresce (glow) when exposed to X-rays and can be identified for removal from the flow of concentrate. They are washed in acid to remove any coatings and sent to Perth for sorting, valuing and marketing.

Alluvial deposits in Australia are strip mined by excavator and either trucked directly to a processing plant or loaded into a mobile screening plant to remove the larger rocks before trucking to the plant. Crushing is not needed, otherwise processing is very similar to that described above. Reject material from the plant is returned to backfill the mine site and the land surface is then comprehensively rehabilitated.

Sorting of rough diamonds by hand begins with separation into about 15 different size groups by sieving. Highly skilled sorters grade these groups into about 7000 different categories according to crystal shape, colour and quality (extent of flaws and impure inclusions). In each group the better diamonds are classified as gems or near gems and the poorer ones as industrial. Automatic machine sorting on the basis of size, colour and shape also is used. High speed sorting machines have been developed in Australia and are used at Argyle to extract the valuable pink diamonds before grading.

Cutting and polishing of a rough diamond may start with 'cleaving' (splitting) a diamond along its grain (weaker crystal planes) to divide it into smaller pieces or remove flaws and impurities. Very thin circular metal blades impregnated with diamond dust or lasers are used to saw across the grain of diamond. A cleaved or sawn diamond is roughly shaped or 'bruted' by grinding its edges off with another diamond. The end result resembles the shape of a spinning-top.

A horizontal spinning metal wheel (scaife) covered with diamond dust is used to grind and polish flat faces (facets) on bruted diamonds. To obtain the maximum gem brilliance the facets must be the right size and at the correct angles to each other. The 'brilliant' cut is the most common for modern jewellery diamonds and consists of 57 facets. Cut and polished diamonds are graded into numerous categories based on clarity, colour, cut and size.

Usually, more than 50% of a rough diamond is lost in cutting and polishing, or up to 80% if it is flawed or misshapen. One hundred and five stones were cut from the largest diamond ever found (about the size of a human fist), called the Cullinan diamond (from South Africa), but they represent only one third of the original crystal weight.

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Uses

The first use of diamond may have been as a talisman or charm by prehistoric humans. Diamond was highly prized as a gem stone in ancient Egypt, Babylon, Mesopotamia and India. The Romans used it to carve cameos from stone and exported diamond to China for cutting jade and drilling pearls. Large diamonds were used by kings, queens and other nobles as symbols of wealth and rank. The two largest stones cut from the Cullinan diamond are part of the British Crown Jewels. The Regent diamond, found in India, in 1701, became part of the French Crown Jewels until stolen in the French Revolution and eventually set in the hilt of Napoleon's sword.

Since the early 1900s diamond has been used to grind and shape very hard metal alloys, such as tungsten carbide, for use as machine tool tips. Diamond polishing powders and compounds are used for fine finishes on optical surfaces, jewel bearings, wire drawing dies, cutting tools, aircraft engine components and many other items made from metals, ceramics, plastics and glass. Diamond grit and powder impregnated rock drilling bits are used in oil, gas and mineral exploration and production; foundation testing for dams, buildings and other construction; masonry drilling; and concrete test sampling. Circular metal saw discs with a diamond impregnated edge also are used to cut roads, grooves in airport runways, concrete, building stone, bricks and shapes for furnace linings.

Diamond machine tools are used in the electrical and engineering industries, such as car, aircraft and ship building. Very thin diamond impregnated saw blades are used to slice brittle metals and crystals for use in electronics. Knives in the home can be sharpened using 'steels' impregnated with diamond powder.

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Suggestions for Further Reading

  • Blainey G 1993 The Rush that Never Ended: A History of Australian Mining, 4th edition, Melbourne University Press, Melbourne.
  • Bruton E 1978 Diamonds, Chilton Book Company, Radnor, Pennsylvania.
  • Australia's Identified Mineral Resources 2007, Geoscience Australia, Canberra.
  • Hughes F E (Ed) 1990 Geology Of The Mineral Deposits of Australia and Papua New Guinea, Australasian Institute of Mining and Metallurgy Monography 14.
  • Woodcock J T & Hamilton J K (Eds) 1993 Australasian Mining and Metallurgy: The Sir Maurice Mawby Memorial Volume, Australasian Institute of Mining and Metallurgy, Monograph 19.
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