Diamonds

The ‘First C’

 

Diamonds are found in the full spectrum of colours. As consumer awareness of fancy coloured diamonds increases, we have enjoyed showing clients the beauty of intense fancy coloured diamonds, including pink, yellow, champagne, green, blue and black. Those exhibiting strong colour are referred to as ‘fancy coloured diamonds’ and are graded on the intensity of their colour.

Traditionally, white (or more correctly ‘colourless’) diamond is favoured. In Australia, we follow the GIA (Gemological Institute of America) grading scale for evaluating colour.

‘D’ coloured diamonds are extremely rare – having no colour at all. Colour grades of ‘D’ through to ‘H’ are referred to as white, ‘I’ colours and beyond are referred to as ‘tinted’ with yellow or brown. Seen in isolation, such a stone may appear to be white, but when compared with a selection of diamonds of higher colour, the subtle differences can be detected.

We find most discerning customers favour high colour and we generally present choices of diamonds graded ‘D’ through to ‘G’.

The ‘Second C’

 

When sufficiently magnified most diamonds will be seen to contain minute traces of (non-crystallised) carbon or other minerals known as ‘inclusions’. The international standard for assessing the clarity of a diamond is the visibility of these inclusions under 10 power magnification. As well as the number and size of inclusions, clarity grading also takes account of their location, nature and colour.

Our preference is to present diamonds with clarity gradings of VS or SI. Beyond this, inclusions can either be detected by eye or can diminish the brilliance of a stone.

The ‘purer’ the diamond, the rarer and more valuable it becomes. Although a little daunting at first, our clients enjoy being taught how to use a 10X power loupe to verify clarity gradings. By so doing they feel more confident to make ‘trade-off’ choices between clarity and the size or colour of the stone. The ultimate choice is a very individual judgement but confidence comes from being carefully guided through this process.

The ‘Third C’

 

The term ‘cut’ is often confused with the shape of the diamond (such as round, princess, pear-shape). However, in diamond grading the ‘cut’ refers to the craftsmanship of the diamantaire: the ability to unlock the beauty within a rough crystal by polishing individual, precise facets.

Ideally an artisan cuts a diamond to make the best use of light. When a diamond is cut to perfect proportions, light is refracted inside the stone, reflected from one facet to another, and then dispersed through the top of the stone. Extensive study of the laws of physics has allowed us to theorise and observe a range of proportions that achieve the maximum return of light.

If a diamond is cut too deeply, some light escapes through the opposite side. If too shallow, light escapes through the bottom of the stone before it can be reflected back to the viewer.

In order to maximise the ‘yield’ of any particular diamond crystal, a cutter may be tempted to depart from the range of ideal proportions.

For an explanation of the way light behaves within a diamond, click here to see an article from our June 2004 Newsletter.

People often comment that our diamonds “sparkle” more. This is due to our determination to choose the best cuts from parcels. The illustration here, of a tray of perfectly cut diamonds, contrasts with a tray of poorly cut stones. Which would you prefer to wear?

The ‘Fourth C’

 

The term carat refers to the unit of weight used to measure all gemstones. It originates from the seed of the carob tree: remarkably uniform in weight and used by merchants to balance scales to weigh gems.

A carat is 1/5th of a gram and is further divided into ‘points’ – 100 points = 1 carat, 50 points (or 0.50 ct) is a half-carat diamond.

The weight of a diamond relates very closely to its size (diameter) but variations can occur if the cutter departs from ideal proportions.

Thus a well proportioned, one-carat brilliant cut diamond should be close to 6.4 mm in diameter. Due to its heavier specific gravity, a round sapphire of the same diameter may weigh around 1.15 ct, while an amethyst of similar size should weigh approximately 0.75 ct.

The ‘Fifth C’

‘Confidence’ in your jeweller.

Knowledge of the ‘4 Cs’ will assist in the selection of the right diamond. However, arriving at the optimum choice will generally involve listening to advice from an expert that you trust and being guided through the process.

At Abrecht Bird Jewellers we enjoy presenting a range of qualities and explaining how differences affect price.

Most of our new business comes from referrals – family or friends who have fond memories of their experiences with us and who recommend us with confidence.

Origin of Diamonds

Diamond is simply carbon superheated under great pressure deep below the Earth’s surface. Carbon is one of the most common elements in the world, and is one of the four essentials for the existence of life. Humans are more than 18 per cent carbon. The air we breathe contains traces of carbon. When occurring in nature, carbon exists in three basic forms: Diamond – an extremely hard, clear crystal. Graphite – A soft, black mineral made of pure carbon. The molecular structure is not as compact as diamond’s, which makes it weaker than diamond. Fullerite – A mineral made of perfectly spherical molecules consisting of exactly 60 carbon atoms. This allotrope was discovered in 1990. Diamonds form about 100 miles (161 km) below the Earth’s surface, in the molten rock of the Earth’s mantle, which provides the right amounts of pressure and heat to transform carbon into a diamond. In order for a diamond to be created, carbon must be placed under at least 435,113 pounds per square inch (psi or 30 kilobars) of pressure at a temperature of at least 752 degrees Fahrenheit (400 Celsius). If conditions drop below either of these two points, graphite will be created. At depths of 93 miles (150 km) or more, pressure builds to about 725,189 psi (50 kilobars) and heat can exceed 2,192 F (1,200 C). Most diamonds that we see today were formed millions (if not billions) of years ago. Powerful magma eruptions brought the diamonds to the surface, creating kimberlite pipes.

Kimberlite Pipes

Kimberlite is named after Kimberly, South Africa, where these pipes were first found. Most of these eruptions occurred between 1,100 million and 20 million years ago. Kimberlite pipes are created in deep fractures in the Earth. The magma inside the kimberlite pipes acts like an elevator, pushing the diamonds and other rocks and minerals through the mantle and crust in just a few hours. These eruptions were short, but many times more powerful than volcanic eruptions that happen today. The magma in these eruptions originated at depths three times deeper than the magma source for volcanoes like Mount St Helens, according to the American Museum of Natural History.

The magma eventually cooled inside these kimberlite pipes, leaving behind conical veins of kimberlite rock that contain diamonds. Kimberlite  is a bluish rock that diamond miners look for when seeking out new diamond deposits. The surface area of diamond-bearing kimberlite pipes ranges from 2 to 146 hectares (5 to 361 acres).

Diamonds may also be found in riverbeds, which are called alluvial diamond sites. These are diamonds that originate in kimberlite pipes, but get moved by geological activity. Glaciers and water can also move diamonds thousands of miles from their original location. Today, most diamonds are found in Australia, Borneo, Brazil, Russia and several African countries, including South Africa and Zaire. Recent finds in Canada are also being developed.

Diamonds are found as rough stones and must be mined before further processing to create a sparkling gem that is ready for purchase.

 Diamonds are mined in 25 countries on almost every continent. Extensive mining of diamonds shifted from India (17th century), to Brazil (18th century), to the African continent (19th century), and finally Australia and Canada (20th century). The top seven producing countries, which account for 80 per cent of the world’s rough diamond supply, Botswana, Zaire, South Africa, Russia, Angola, Namibia and Australia (now ceased production and mine is now closed). Russian diamond discoveries were made during the 1950s in Siberia, a most inhospitable mining area where the permafrost is up to 300 metres deep. The Argyle diamond story started in the early 1970s, when one of the world’s most significant finds was made at Smoke Creek in the remote north of Western Australia, over 2000 kilometres from Perth. these deposits were traced subsequently to the primary pipe known as AK1 in the Kimberley – Lake Argyle region. Even though gem-quality production from the Argyle has ceased, it was the world’s biggest producer of natural diamonds and contributed approximately one-third of the world’s natural supply. The recovery ratio of diamonds is incredibly low. It takes more than thirty tonnes of diamond-bearing ore to produce a single, polished diamond of one carat – a diamond weighing one-fifth of a gram! These days, diamonds are more likely to be found by sophisticated techniques including satellite imaging than random discovery by a boy kicking at stones on a riverbank.

 

Diamond Pipes

Millions of years ago, natural carbon was compressed into diamond crystals by great forces of heat and pressure deep within the earth. Later, the molten magma in which the diamonds were formed was forced to the earth’s surface, through fissures such as the throats of ancient volcanoes.

The molten material then cooled into formations called ‘pipes’, composed of an igneous rock called kimberlite or of lamproite. The common term for both kimberlite and lamproite is ‘blue ground’.

Types of Diamond Mining

There are three main types of diamond mining, each depending on the nature of the deposit.

Primary Deposits (at the location of the pipe)

The Open Cast Mine

Initially, all pipes were worked by simple, open-cast (open-pit) methods. Depending on conditions, the maximum depth for opencast mining is generally considered to be 245-305 m.

Today’s open-pit mining is a far cry from South Africa in the 1870s, when thousands of independent claim-holders crowded into the Kimberley Mine and eventually hindered and endangered the entire operation as the depth of the excavation increased. The great risk then was the collapse of surrounding rock into the mine. In modern pits, the reef is gradually cut back in steps as the mine goes deeper – resulting in a wider, deeper, safer hole.

The Underground Mine

Underground mining was not successful in South Africa until 1890, after all the independent claims finally were consolidated into the ownership of De Beers. For this style of mining, vertical shafts are sunk into the ‘country rock’ at a distance from the pipe. There are various methods, but the basic technique is to drive horizontal tunnels leading into the pipe from the vertical shafts, and then to dig out the diamond-bearing ore from below. The hole is made progressively deeper, and the tunnels are driven at successively lower levels, until the entire pipe has been mined out so that only a hole remains.

Secondary Deposits

Erosion of diamond pipes is the source of all the widely distributed alluvial diamonds that have been swept away by streams and rivers to the places where they are now found.

Prospecting for payable alluvial deposits is not an easy task and the ratio of diamonds to waste material is extremely low.

Until the discoveries in South Africa in the latter half of the nineteenth century, diamonds were recovered entirely from alluvial deposits. Methods were simple and labour intensive, relying on a pick and shovel for digging, a hand-held washing pan to separate diamonds from gravel, and hand sorting.

Miners from all parts of the world brought their knowledge to the new fields in South Africa, and they applied gold-mining techniques to the recovery of diamonds from alluvial gravels.

These diggers introduced basic methods of alluvial diamond recovery which have changed only in scale and sophistication in the decades that followed.

Marine Deposits

Marine deposits are a variation on alluvial deposits, resulting from the wave action of the ocean, concentrating diamonds at the base of the surf zone. Waves arriving at an angle to the coast tend to push the diamonds along the coast, causing the diamonds to stream out from where rivers deposited them at the coastline.

There are three types of marine mining operations.

1. Sand is moved from 10 metres below sea level to as far inland as the sea may have risen, in order to reveal the concentrations on the bedrock.
2. Divers and boats work in the surf zone to 20 metres of water and use suction pipes to remove gravel and diamonds from the ocean floor.
3. Deep-sea marine vessels use remote underwater tractors or large excavators to remove overlying sediments and extract the diamond-bearing sand and gravel.

In 1852, the term fluorescence was first used to describe the reaction of UV light to the mineral calcium fluoride, otherwise known as Fluorite. In gemmological terms, fluorescence is the emission of light from gemstones (and diamonds in particular), when they are exposed to ultraviolet light. Thought to be caused by sub-microscopic defects in the crystal lattice of some gems, this optical effect is caused by the excitation of atoms after exposure to ultraviolet light. Resulting colours can be yellow, white, pink and orange, but most commonly, blue is displayed. Approximately 30–35% of diamonds fluoresce to some degree.

 

 

World’s hardest and oldest diamonds from Australia

 

The oldest-known diamonds, almost as old as the Earth itself, have been found in Australia and could hold the key to explaining  how the planet’s crust evolved. The four-billion-year-old diamonds were found trapped inside zircon crystals from the Jack Hills region, hundreds of kilometres north of Perth. The diamonds are thought to be about one billion years older than any found in terrestrial rock.

The Argyle region of Western Australia is likely to be the most well known source of Australian diamonds, but they have also been found in the New England area in New South Wales. Crystals found in this region are recognised as the hardest in the world. They are generally small, perfect to semi-perfect octahedra and are used to polish other diamonds.

This hardness is considered to be a product of their single-stage crystal growth form. Most other diamonds show more evidence of multiple-growth stages that produce inclusions, flaws and defect planes in the crystal lattice, all of which affect their hardness.

Although known to be the hardest naturally occurring mineral, diamonds can chip or break.