Diamonds originate as carbon particles in the Earth’s mantle, which are subjected to such high pressure and temperature that they crystallize to form diamonds. Millions of years ago the molten rock, or magma, in the mantle burst through weak points in the Earth’s crust as volcanoes, and it is in the pipe of cooled magma – called kimberlite, after Kimberley – that diamonds are found. Finding kimberlite, however, isn’t necessarily a licence to print money – in every one hundred tonnes there will be about twenty carats (4g) of diamonds.

The word “carat” derives from the carob bean – dried beans were used as a measure of weight. (Carat has a different meaning in the context of gold, where it is a measure of purity.) De Beers estimates that fifty million pieces of diamond jewellery are bought each year – which represents a lot of marriage proposals.

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