DIAMOND MINING

A diamond is essentially just carbon, one of the most common elements. But we usually find carbon compounded with other elements, and when pure carbon does occur on the Earth’s surface it takes the form of graphite. Making diamond requires much higher temperatures and pressures – those which are only found inside the Earth’s mantle at depths of a hundred miles or more. There may be lots of diamonds there – perhaps as much as a quadrillion tonnes – but they’re totally inaccessible to us.

The only way to get mantle material up to the surface is through a volcanic eruption, and today this simply doesn’t happen deep enough to bring up diamonds. But billions of years ago, when the Earth was hotter, such eruptions did occur.

They resulted in vertical columns of rock that originated deep in the mantle, and some of these – called kimberlite pipes – contain diamonds.

Where kimberlite is exposed on the surface, it erodes like any other rock, and for centuries the products of this erosion, typically found in riverbeds, were the only source of diamonds. Then, around 1867, a 15-year-old boy named Erasmus Jacobs found a diamond on the bank of the Orange River in South Africa. Subsequent investigations led to the discovery of the first solid outcrops of kimberlite, named after the mining town of Kimberley that grew up around them.

Kimberlite pipes can be mined in two ways: either underground via tunnelling, or in open-pit mines where any overlying layers have been removed. This may sound analogous to coal mining, but whereas coal is solid rock that can be broken up and used almost immediately, diamonds make up only a tiny proportion of kimberlite. Around 250 tonnes of ore needs to be mined to produce a single gem-quality diamond of just one carat, or 200 milligrams.