Beautiful and useful: understanding diamonds

Diamonds. If you work in a busy open plan office chances are you are only several metres away from someone wearing one. You may even be lucky enough to be wearing one yourself. You won’t find them just at jewellers; your local hardware store will also stock diamond drill heads. 

Diamonds are both beautiful and useful and both the natural and synthetic are known to be the hardest ‘accessible’ material on Earth. Well technically, a laboratory made nanomaterial has claimed the ‘hardest’ substance billing but our quandary is if diamonds are so hard how do they cut and shape diamonds into spectacular shapes? If they are pure, packed carbon then why do some have a tinge of brown, pink, blue or yellow?

Professor David Phillips from the School of Earth Sciences says that in short, to cut a diamond you simply need another diamond.

“Jewellers will create a notch with another diamond and cleave it by tapping it with a steel blade. The reason is diamonds grow as crystals and carbon atoms are arranged into a cubic framework structure. While this arrangement makes a diamond very hard, it’s also somewhat brittle. This is because there are weaknesses along the cubic planes.

“Diamonds are also cut using a Phosphor-bronze blade impregnated with diamonds and rotated at very high speeds. It is a slow process and the diamond glows red even with cooling fluid flowing over it.

“Special lasers have also been useful to cut diamond especially if they are irregular as they can shatter when being cut.”

The brilliant cut is the favoured way to cut and facet gem-quality diamonds in order to maximise light reflectivity from the facets. 

“The brilliant cut also utilises the natural octahedral shape of most diamonds, so the cutter can take advantage of the planes of weakness to cleave the diamond and minimise cutting and polishing time. Brilliant-cut gem diamonds are common in diamond rings.”

As for the colour spectrum of diamonds, Professor Phillips says diamond usually comprises more than 99.7 per cent carbon. 

“The remaining 0.3 per cent constitutes traces of impurities of other elements. If you look at the periodic table of elements, to the left of carbon is boron and to the right is nitrogen. If there are traces of nitrogen, the colour of the diamond is yellow, traces of boron make it blue. Some diamonds are brown and these have experienced a rough time in the planet’s mantle. At the molecular lever the planes of weakness have slipped or deformed. This means light penetrating the diamond is slightly distorted, resulting in the brown colour. The pink colour is a conundrum, but it probably also relates to plastic deformation of the diamond to a degree that gives it a pink hue. Black or grey diamonds may contain abundant inclusions such as graphite or sulphides and are often composed of many small diamonds intimately inter-grown with one another.” 

Natural diamonds are old minerals and can be millions or billions of years old. They are formed under extreme pressures and temperature below the Earth’s surface. 

Professor Phillips says most diamonds originate from depths of around 150 to 200 kilometres in the mantle, reaching the surface of the Earth via erupting volcanoes. 

“As the magma travels up to the surface at close to the speed of sound it rips off pieces of the mantle taking the diamonds with it,” he says. “The diamonds of different sizes and quality are embedded inside the crystallised host rocks referred to as kimberlites and lamproites. The Argyle mine in Western Australia harvests lamproites.”

It’s amazing to think that we can now recreate extreme temperature and pressures conditions to make synthetic diamonds, and that there is a company that offers the service of transforming the cremated ashes of a loved one into a diamond.

“My research studies the formation, composition, and therefore location of the host rocks that contain diamonds,” Professor Phillips says.

“Understanding the tectonic processes that control the emplacement of these volcanic rocks gives us insights into their genesis and where we can find them, which is especially important for the mining industry. 

www.earthsci.unimelb.edu.au/research-area/noble-gas-geochronology-geochemistry