Thanks to the rise of feminism, it has ceased to be acceptable to regard them as a girl's best friend. So what are diamonds? Everyone knows that they are a form of carbon, but just how they got their distinctive hard and dense form is a story that tells us a lot about the deep Earth.
There are two main forms of carbon, diamond and graphite, along with some rarer types including the Fullerenes. In graphite, the atoms are arranged in hexagons making up flat sheets. That is why it comes off when you rub it on paper. Mixed with clay and other ingredients, it is the “lead* used in pencils, despite not containing any lead. By contrast, diamond consists of carbon atoms in a three-dimensional structure that they only adopt under extreme pressure. Because every atom is bonded to four others, the result is a very hard crystal.
The artificial diamonds used in drilling, and in jewellery for someone you are not terribly keen on, are made by squeezing carbon in a mechanical press until it changes phase to the more compact diamond form. Diamonds have a density of about 3.5 grams per cubic centimetre, compared to graphite's 2.3, so applying pressure is bound to make graphite turn to diamond at some point.
To achieve this effect inside the Earth, you need only take graphite to a depth of about 120km. There the pressure is about 40,000 times that of the Earth's atmosphere at sea level and the temperature is up to about 900°C, which also speeds up the process. It appears from analysis of the impurities they contain that diamonds can form from rocks that are common in the Earths mantle such as peridotite. They also tend to be very ancient, between 1 and 3.3 billion years old.
But once a diamond has been made it has to get to somewhere near the Earth's surface for us to find it. This involves one of the most violent events in nature, a type of volcanism which has been seen at its most abundant in South Africa and is called a Kimberlite volcano, after Kimberley in South Africa.The volcanic material cuts its way up through the Earth's mantle and crust at a pace which finally becomes near-supersonic. No Kimberlite eruption has ever been seen; if one did occur it would be a major natural disaster. For blushing brides, however, they are absolutely essential. A diamond brought slowly to the surface from the mantle would gradually revert to graphite on the way up. But bringing them to the surface at this speed freezes them in diamond form. Although mineralogists point out that all the world’s diamonds are gradually reverting to graphite, the timescale involved is billions of years.
Even in Kimberlites, diamonds are rare and thousands of tonnes of rock are shifted to find a few kilograms of them. They are also extracted from rivers that erode Kimberlites.
Very small diamonds can also be created by other high-pressure events in nature. They are sometimes formed during metamorphism inside the Earth, and they are found in meteorites as evidence of shock waves in deep space.