What are diamonds?

Diamonds are crystallised forms of carbon that predate the oldest known life on Earth. Except for trace impurities, a diamond is composed solely of carbon, the chemical element that is fundamental to all life. Assembled in the most tightly bound crystalline configuration, a diamond is the hardest structure known to man; a diamond can only be scratched by another diamond.

This unique durability ensures diamonds’ heritage; as jewellery, they can be worn every day for a lifetime, then passed as valuable heirlooms to following generations.

How were diamonds formed?

Diamonds were formed billions of years ago, deep within the earth. Their formation required very specific conditions, involving the exposure of carbon-bearing materials to a high heat and enormous natural pressure. These conditions occur naturally in only one place on Earth, in the layer known as the mantle, approximately 150 to 200 kms from the earth’s surface, where a combination of high pressures and temperatures exist. Most diamonds are more than three billion years old, two-thirds the age of the Earth.

Ascent to surface – kimberlite pipes

Diamonds were carried towards the earth’s surface in rare eruptions of molten rock, or magma, which originated at great depths. This magma emanated from deep cracks and fissures and erupted in small but violent volcanoes. It rose to the surface and created a carrot-shaped “pipe” filled with volcanic rock, minerals, mantle fragments and sometimes embedded diamonds. These pipe formations are called kimberlites after the city of Kimberley, South Africa, where they were first discovered in the 1870s.

Only a small number of diamonds survived the journey to the earth’s surface; fewer still are of gem quality that can be made into precious pieces of jewellery. In spite of the fact that more than 12 000 kimberlite deposits have been found worldwide in the last 25 years, fewer than 1% have contained enough diamonds to be economically viable.

Not only do kimberlite pipes give diamonds to the world, but they also serve as a window into the centre of the Earth; there is probably no other group of rocks presently found at surface that originated from as great a depth.

Lamproite pipes

Lamproite pipes are another source of ‘primary’ or ‘hard rock’ diamonds, but to a much lesser extent than kimberlite pipes. Lamproite pipes are created in a similar manner to kimberlite pipes, except that boiling water and volatile compounds contained in the magma act corrosively on the overlying rock, resulting in a broader cone of eviscerated rock at the surface. This results in a martini-glass shaped diamondiferous deposit as opposed to a kimberlite pipe’s champagne flute shape. There are relatively few lamproites and only olivine lamproites are diamondiferous; other varieties, such as leucite lamproites, are presumed not to originate deep enough in the mantle to contain diamonds. Even amongst the olivine lamproites, few contain diamonds in economic concentrations.

Alluvial diamonds

Alluvial diamonds have been removed from the primary source (kimberlites or lamproites) by natural erosive action over millions of years, and eventually deposited in a new environment such as a river bed, an ocean floor or a shoreline. The locations of these alluvial diamond deposits are controlled by the surrounding topography, drainage patterns, and the location of the kimberlites themselves. Alluvial deposits are often mined and exploited by small-scale miners using artisanal mining techniques. Organised alluvial mining, on the other hand, is carried out by large companies using heavy equipment. In this sort of mining, millions of tons of sand are removed from river beds and the coastline. The silt, including rubble, is removed for sifting and separation.

Marine diamonds

Diamonds that were transported downstream, but did not remain deposited on land, made their way to the sea bed just offshore. Diamonds in marine areas are typically trapped in bedrock depressions such as gullies, potholes, depressions, channels or other trapsites.

Where are diamonds found?

It is generally accepted that diamond deposits are found in the oldest parts of continents called ‘cratons’, where the basement rocks are older than 1.5 billion years.

These cratons can be divided into two terranes: Archean-age archons, which are older than 2 500 million years, and Proterozoic-age protons, which are 1 600–2 500 million years old.

How diamonds are found: exploration

Generally, kimberlites occur in clusters of up to 5 or more, in close proximity to each other. They are not necessarily all the consequence of a single volcanic event. Indeed, they may have resulted from several different events over a period of time, adding to the complexity of sampling and proving their economic potential.

Geologists use many methods to explore for kimberlites, including satellite remote sensing, geophysics and reconnaissance sampling, before drilling the kimberlites themselves to establish whether or not they contains economic quantities of diamonds.

The first step is generally to investigate areas with a history of diamond recovery, and then to follow up with stream or deflation sampling for evidence of kimberlite indicators such as garnets. Thereafter, the use of geophysics to search for magnetic anomalies is applied. Sampling and drilling are then used to confirm whether the anomalies are indeed kimberlites.

Once an anomaly has been confirmed as a kimberlite, heavy mineral analysis (HMA) sampling of representative material is carried out as a quick and efficient method of assessing whether the kimberlite has the potential to be diamondiferous. Micro-diamond and mini-bulk sampling are then used to establish if there is the potential for those kimberlites prioritised by HMA sampling to host an economic concentration of diamonds.

If positive results are achieved through the initial processes mentioned above, then a company will be in a position to commence bulk sampling material. The aim is here to establish the economics for mining a particular kimberlite, and will give indications of grade, cost per ton and average value per carat. From here, a production decision can be made.

How diamonds are recovered: mining

Hard rock mining

Mining of a diamond-bearing pipe starts with the excavation of a pit into the kimberlite pipe. In this process, called “open-pit” or “open-cast” mining, the initially weathered ore material is removed with large hydraulic shovels and ore trucks.

Hard rock is drilled and blasted with explosives so the broken material can be removed. When deep ore warrants it, the mining goes underground with vertical shafts descending to horizontal drifts, or passageways that enter the pipe. In bedrock adjacent to the pipe, shafts are sunk and drifts are tunnelled into the pipe.

Concrete-lined tunnels are excavated under a large vertical section, perhaps 100 to 200 metres of kimberlite. Along the tunnels are draw points, or openings in the concrete casing where kimberlite is drilled and extracted so as to destabalise the underlying column whereafter it collapses (‘caves’) and is withdrawn from the created draw points. This is known as ‘caving’.

Broken kimberlite falls through the draw points and is scraped out of the tunnel with a drag or scraper bucket attached to a cable and winch, working much like a clothes line on a pulley. The kimberlite above the tunnels falls under its own weight and leads to a slow, continuous caving ground that is removed through the draw points.

The scraped kimberlite rubble is loaded into cars on a lower level and moved to a crusher underground. The crushed ore is then conveyed to skips that carry the ore up the vertical shaft for processing.

Fissure mining

Erosion will eventually wear down a kimberlite ore body until only the root zone remains, leaving only this narrow ‘fissure’ zone of magmatic kimberlite to be mined. These fissures are vertical orebodies that are generally very narrow, with an average width of less than 70 centimetres. Fissure mining, otherwise known as ‘dyke mining’, yields much lower tonnages and is generally complex because the ore body is narrow. More than one type of kimberlite can be present within dyke systems due to multiple intrusive phases varying in both space and time. This, too, can further complicate the mining process. Few companies have successfully tackled fissures for long periods, and Petra is regarded as the world leader in the fissure mining field.

Alluvial mining

Alluvial mining is a more straight-forward process. Operators strip the overburden to uncover the river or marine gravels, which are then mined using a conventional open-pit method. Processing alluvial ore is cheaper because it generally does not require blasting or major crushing before being sent to pan plants or dense media separation (DMS) plants.

In the mining, transportation and treatment of river gravels, only the strongest, least-fractured diamonds survive. This means that alluvial diamonds are normally of higher quality than run-of-mine kimberlite diamonds, although grade is usually low.

Processing

Once a diamond operation yields ore, the diamonds must be sorted from the other materials. Excavated ore is transported to a processing plant.

Hard rock ore (kimberlite) is first crushed and then processed through the plant, , which consists of a series of screens, jigs and scrubbers and a gravity pan or DMS plant to remove lighter particles and create a concentrate of heavy material, which includes the diamonds.

Diamonds are then extracted by using an X-ray machine and/or grease table and checked by hand sorting. Most diamonds luminesce under X-rays and can be identified and separated in final recovery. However, some diamonds – particularly more valuable type II stones – do not respond well to X-rays, so grease tables are used to recover the diamonds. As diamonds are hydrophobic, meaning they repel water, they stick to the grease while the rest of the wet concentrate runs off.

Waste material is stored in tailings dumps. Some of these may contain economic grades due to early, inefficient processing of high grade kimberlites and alluvial deposits. The opportunity exists to reprocess tailings dumps in order to recover those diamonds that were missed first time around.

Sorting and distribution

Once mined, rough diamonds are delivered to sorting experts who categorise and assign a value to them – no mean feat considering that no two diamonds are the same. Diamonds are sorted into parcels according to their shape, size, clarity and colour, but within these categories there are thousands of variants which can affect the price. It is at this point that gem quality diamonds are separated from industrial diamonds. Otherwise known as ‘boart’, industrial diamonds are small, lower-quality stones that can be used in equipment such as drill bits and lathes.

Traditionally, the vast majority of diamonds were sold through De Beers’ centralised selling channel, the Central Selling Organisation (CSO), but it is now common for many companies, such as Petra Diamonds, to sell their goods via tender. Buyers known broadly as diamantaires, often members or representatives of families that have dealt in the purchase, cutting and polishing and sale of gem-quality diamonds for generations, attend these auctions at any one of the 24 registered diamond bourses around the world and place competing bids to ‘win’ the diamond parcel of their choice.

Cutting and polishing

The process of transforming a rough diamond into a polished gemstone is both an art and a science. A well-cut diamond reflects light within itself, from one facet to another, as well as through the top of the diamond, bringing out its spectral brilliance.

The cutting and polishing of a diamond crystal always results in a dramatic loss of weight; rarely is it less than 50%. Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the carat rating.

After a stone has been cut, it is then polished and classified again, this time by its cut, colour, clarity and carat weight, characteristics known as ‘the four Cs’. It is then sold via one of the registered diamond exchanges (also known as; bourses’) located around the world or directly to wholesalers or diamond jewellery manufacturers.

Retail

Diamonds have come to represent the ultimate gift of love and commitment, and it is therefore fitting that the cornerstone of diamond jewellery sales worldwide is the engagement ring. Whilst diamonds qualify as luxury goods, they are much more than this, expressing as they do powerful human emotions.

For hundreds of years, diamonds have been given to celebrate the most important moments in people’s lives, such as engagements, weddings, anniversaries and the birth of a child. All over the world, thousands of diamond retailers cater to these needs, from small, independent jewellers to mass market superstores, and there has recently been huge growth in sales online. Purveyors of antiques also have an important place in the market given that the value of a diamond endures and appreciates over time.

As diamonds continue to broaden their socio-economic and geographical appeal, designers duly respond and there is a diamond size and style to suit every taste.

Coloured diamonds

While coloured diamonds have been known and admired for centuries, their phenomenal growth in widespread popularity is a relatively recent phenomenon. In the not so distant past only large historic diamonds such as the Hope blue or the Dresden green commanded much attention, even within the gem and jewellery community. A major turning point came during the late 1970’s when the Argyle mine (Rio Tinto) in Western Australia was discovered, a prolific source of brown (“champagne”) and yellow diamonds, as well as some pinks and greens. New availability of coloured diamonds meant that they became more accessible to the greater public and Australian marketing transformed the previous mindset that colourless diamonds are most desirable.

More attention was brought to these precious stones in 1987 when Christies auctioned the Hancock Red, a 0.95 carat red diamond, in New York. It was sold for $880,000, the record price per carat for any gem ever sold at auction at that time. However currently the record for the most expensive diamond ever sold belongs to a 3.73 carat blue diamond sold at auction to Lawrence Graff in May 2008, at US$1.3 million per carat.

According to Daniel Prince, a bespoke London jeweller, ‘No other jewel combines the rarity, beauty and sex appeal of a coloured diamond’. Coloured diamonds are indeed exceedingly rare. For every 100,000 D-flawless diamonds, there is perhaps one coloured diamond, and it is probably not flawless. The beauty and the rarity of these coloured diamonds have generated unprecedented desire and incomparable prices.

A diamond gets its colour from minute amounts of trace elements, such as nitrogen, boron, graphite and hydrogen, which interact with the carbon atoms that make up the stone, or via exposure to natural radiation. Each element contains its own hues. Nitrogen will result in a yellow or orange stone, which, with additional amounts, becomes deeper in colour. Sub microscopic inclusions, such as graphite, block all transmissions of light, resulting in a black diamond. Boron produces blue and uranium develops green. Hydrogen creates pink, purple and red. According to François Curiel, jewellery expert and chairman of Christie’s Europe, "Rarely do imperfections add value. In the case of coloured diamonds, it is ironic that the beauty and rarity of these gems comes from an impurity. While a perfect colourless diamond now trades at $100,000 per carat, the benchmark for the best colour diamonds has surpassed $1 million a carat and continues to rise."

Colour is the most important factor in determining the value of a gemstone, this being determined by its hue, tone, saturation and distribution. Hue is the term used for the actual colour of the spectrum: red, orange, yellow, green, blue, indigo or violet. The more pure a gemstone's hue, the more valuable it is. Because gemstones are composed of many naturally occurring elements, they typically emit one dominant colour and one or more underlying colours. Tone represents how light or dark a stone appears depending on how much brown, black, grey or white is present. Saturation describes strength of colour in terms of degrees of intensity- the more colour saturated a gemstone is, the more valuable it becomes. Distribution is how evenly the colour spreads out across the body of the gemstone.

Coloured diamonds are graded by the terms Fancy, Fancy Deep and Fancy Vivid, in ascending order of desirability. Fancy Deep describes stones of medium to dark tone and moderate to strong saturation while Fancy Vivid is a deep colour with no infiltration of other colours. According to Thomas Burstein, "The main concern for collectors is that the diamond shows its colour; it has to be evenly dispersed."

Each coloured diamond is different not only because of its natural colour but also because it is shaped and finely polished. The cutting of coloured diamonds is significantly important, performed by highly skilled craftsmen who combine their technical knowledge with a deep appreciation for beauty and colour.

Blue diamonds

Natural blue is among the rarest of all coloured diamonds and these stones are therefore highly sought after by collectors. The colours can range from faint to a very deep blue. In addition to their unique colour attributes, blue diamonds have distinctive traits such as electrical conductivity, boron as the colouring agent and a characteristic phosphorescent reaction to shortwave UV.

India was the main producer of blue diamonds from the 16th to the 18th centuries. This was the source for the 112.25 French blue that later became one of the most infamous diamonds ever, the 45.52 carat Hope diamond. The Hope diamond is a large, deep blue diamond, currently housed in the Smithsonian natural History Museum. According to legend, a curse was place the large, blue diamond when it was stolen from an idol in India - a curse that foretold bad luck and death not only for the owner of the diamond but for all who touched it. It subsequently passed through many hands, including that of King Louis IV who was executed in the French revolution. The Hope diamond appears to be a brilliant blue to the naked eye and exhibits red phosphorescence under ultraviolet light.

Today, the only reliable source of blue diamonds is Petra Diamonds’ Cullinan mine in South Africa, and Petra sold a 39.19 carat Cullinan blue in October 2008 for US$8.8 million. Other South Africa mines reputed to have produced blues in the past include the Jagersfontein (De Beers), Koffiefontein (Petra), the Bellsbank mine near Barkly West and the Helam mine (again Petra) at St. Swartsruggens. Very rarely, blue diamonds have been found in alluvial deposits at Lichtenburg in the Western Transvaal. Guinea and western Africa are also known to have produced blue diamonds. Historically, central Africa has been rumoured to be a source of blue diamonds, especially Sierra Leone for lighter blues.

In November 2008, a blue diamond became the most expensive gem ever sold, when the Wittelsbach diamond sold for US$24 million at a Christies auction in London. A Christies spokesperson commented about the 35.56 carat cut diamond: "Blue diamonds are rare and to offer a blue diamond of this size, quality, shape and provenance is truly extraordinary."

Pink diamonds

Pink diamonds are known to occur in only a few mines throughout the world, and none of these have ever proved to be a steady commercial source for gem-quality pinks. It is this scarcity, coupled with their beauty, that has made them a highly desirable in the international jewellery market. Celebrities such as Jennifer Lopez, who sported a 6.1 carat pink diamond engagement ring in 2002, have helped to spread their appeal yet further.

The alluvial deposits in southern India produced a limited amount of pinks during the active mining years of the 17th century. The many alluvial deposits throughout Brazil have also been a notable but infrequent source of pinks. Currently the Argyle mine (Rio Tinto) in Australia is the world’s most significant source of pinks, though several Russian and African deposits, including the Williamson mine in Tanzania (owned by Petra Diamonds), are also known sources. Williamson is the source of the famous Williamson Pink, a 54.5 carat rough diamond which was recovered in 1947. It was polished to form a round ‘brilliant’ cut of 23.6 carats, and was presented to the then Princess Elizabeth as the centre piece of a floral brooch for her forthcoming wedding to Prince Phillip. Pinks have also been recovered at Petra’s Koffiefontein mine in South Africa.

The Darya-i-nur-"Sea of Light", "River of Light" or "Ocean of Light “diamond is the largest pink diamond and one of the largest and oldest diamonds in the world. Weighing 182 carats its colour, pale pink, is one of the rarest to be found in diamonds. The Darya-i-nur presently forms part of the Iranian Crown Jewels.

Yellow diamonds

Fancy yellow diamonds have four colour grades; Fancy Light Yellow, Fancy Yellow, Fancy Intense Yellow and Fancy Vivid Yellow with increasing prices the higher the intensity of the colour. Some of the yellows with the higher intensity of colour are as rare as pinks and blues and command unusually high prices.

Although faint yellow in white diamonds is not desirable, fancy intense yellow diamonds have long been recognised and prized among collectors. As with other colours, yellow diamonds have come to the public’s attention through the interest generated by a number of special stones, such as the historical 128.54 carat Tiffany. The Tiffany yellow diamond is one of the largest fancy yellow diamonds ever discovered; it weighed 287.42 carats in the rough when discovered in the Kimberly mine, South Africa, in 1878. The stone was purchased by New York jeweller Charles Tiffany and his gemmologist, George Frederick Kunz, studied the gem for a year before beginning to cut it into a cushion shape of 128.54 carats with 90 facets - 32 more than a traditional round brilliant. The diamond is known to have been worn by only two women during its lifetime. It was worn by Mrs. Sheldon Whitehouse at the 1957 Tiffany Ball. It was subsequently worn by Audrey Hepburn.

Common names for yellow diamonds include canary, cape, daffodil, lemon, autumn, golden, manila, chartreuse, saffron etc. Nitrogen in abundance is a major cause of yellow in diamonds. It occurs in higher concentration than any other element. The secondary hues and colour modifiers for yellow diamonds include orange, green, brown, grey and olive. The yellow diamond rough is mined in South Africa, Brazil, Russia and India.

Red Diamonds

Reds are undoubtedly the rarest of coloured diamonds. Less than 20 stones have so far ever been certified as a red diamond, most of them weighing less than half a carat. These diamonds are a collector’s item and every year prices reach new records. Prices per carat have so far ranged from about US$800,000 to US$1.9 million which makes red diamonds one of the most concentrated forms of wealth. Furthermore, they have never significantly lost value because supply has never come close to exceeding demand.

Red diamonds have the same properties as pink diamonds; the term ‘red’ refers to a dark or intense pink. Therefore the only colour grade for a red is ‘fancy’ as it falls in the pink category when being any lighter. They have been found in Brazil, South Africa, Borneo, India, Venezuela and Australia.

The most famous of these coloured stones is the Moussaieff Red Diamond. It is the largest red diamond ever to be graded by the GIA. The Moussaieff Red is reported to have been found by a Brazilian farmer in the mid-1990’s as a rough of approximately 13 carats. The diamond was purchased and cut by the William Goldberg Diamond Corp. where it went by its original name the Red Shield. The now 5.11 carat gem is currently owned by Moussaieff Jewellers Ltd.

Common names for red diamonds include blood red, ruby, magenta, raspberry, rose, strawberry, cherry, tomato etc. Red diamond rough is mined in Australia, Brazil and South Africa.

Green Diamonds

Green diamonds with no other secondary hues or modifiers are some of the rarest and depending on intensity and purity of colour, command excessive prices. Green colour in diamonds is caused by exposure to the natural radiation in the earth such as uranium ore. The presence of brown or green radiation stains on the surface is indicative of the natural green colour, though very few diamonds available for the examination process can be proved to be of natural origin.

Dresden Green

The 40.7 carat Dresden Green is the largest and perhaps the finest green diamond known to have a colour of natural origin. It is not only of extraordinary quality but also a very rare type lla - one of the purest forms of diamond. The Dresdon is believed to have come from India, though it is often argued to be of Brazilian origin, and was purchased by Frederick Augustus the Second from a gem merchant at the Leipzig Fair in 1742. Since then, it has been exhibited for public display in the west wing of the Dresden castle.

The common names of green diamonds include: emerald green, bottle green, forest, grass, chartreuse, teal etc. The green rough diamonds are mined in Brazil, South Africa, India, Australia, Congo, Ghana and Siberia.

Sources:

In Review, Colored Diamonds. John M. King (ed.) California 2006; Diamond Circle Capital Plc. September 2008; www.danielprince.co.uk