Diamond is a naturally occurring mineral with an extraordinary crystalline structure and strong chemical atomic bonds that account for its strength and durability. The carbon atoms that form diamond are released when the rocks carrying them melt under intense heat. These atoms, given an environment of the right levels of heat and pressure, bond together and start to form diamond crystals. It is, however, a very delicate process since a small increase in temperature causes the crystals to melt, and a drop in temperature halts their growth.
Diamonds were formed at least 990 million years ago, although some are estimated to be as old as 4.25 billion years, older than life on planet earth. They were crystallised in various types of rock, under enormous pressures of 45–60 kilobars, at temperatures of 900–1000 centigrade, and at depths of 125–200 kilometres below the earth’s surface. Some rare specimens developed at depths of 300–400 kilometres.
Kimberlite, or diamond-bearing ‘pipes’, are formed when magma, or molten rock, forces cracks into the surrounding rock in the earth’s upper mantle. Carbon dioxide bubbles in the magma expand due to heat below or reduced pressure above, causing the magma to explode upwards through the cracks, forming cone-like pipes leading to the earth’s surface. Diamonds, along with other minerals from deep within the earth, are carried up with the magma, as passengers. This happens so quickly, at the speed of sound, that diamonds are not able to convert to graphite – the stable form of carbon – and once the diamonds cool, they lack sufficient energy to re-form their crystal structure into graphite. The volcanic cone eventually cools and magma hardens into kimberlite, which is eroded and weathered by the elements.
Diamondiferous pipes of mineral-rich volcanic rock, kimberlite or lamproite, are known as primary deposits. Diamond-containing deposits that have travelled some distance from their original source are called secondary deposits. For example, diamonds in Namibia have travelled over one thousand miles from their source in South Africa, transported by the Orange River.
Miniscule diamonds, or ‘nano diamonds’, have also been found in meteorites, older than the solar system. They could possibly have come from a supernova, literally stardust, which ties in with the myth that diamonds were the tears of the gods. Here science and poetry meet.