There is nothing quite like a diamond. For many, they are the ultimate ‘I love you’ gift, and jewellers will tell you that the ultra-hard stones have unrivalled ‘fire’ and ‘sparkle’. The sentimental and aesthetic value of the gems is matched by their price, which can run into tens of thousands of dollars per carat – and even more for coloured diamonds, especially if they are blue, green, purple, orange, red or pink.
So why are diamonds so expensive? How are they formed? Are diamonds really found in volcanoes? How do they relate to supercontinents and ancient life forms?
In new research published in Nature Communications, we answer some of these questions by studying the world’s largest diamond deposit, Argyle in Western Australia, the source of more than 90% of pink diamonds.We found that at Argyle, diamonds crystallised deep in the Earth’s interior were brought to the surface when a supercontinent, Nuna, began to break apart. As continents break apart, their edges stretch, allowing small pockets of diamond-rich magma to rise to the surface.
What makes pink diamonds so special?
Only about 20% of mined diamonds are gem quality. If you think of diamonds as cars, 80 out of every 100 on the road would be old, beat-up rides and 20 would be luxury cars.One in 10,000 would be the equivalent of a supercar: a rare and precious coloured diamond.
But some places in the world are special. Just as you might see a higher proportion of supercars in Monaco or Hollywood, some places produce more coloured diamonds.When it comes to pink diamonds, one place stands alone. More than 90% of all pink diamonds ever found come from a single mine in the Kimberley region of Western Australia: Argyle.
The Argyle mine will close in 2020, and the price of pink diamonds has soared as the supply dwindles.While pink diamonds are highly prized, they are also damaged goods.Diamonds are made up of carbon atoms arranged in a compact, regular lattice. Clear, perfect diamonds sparkle because light reflects off their inner surfaces.
But when diamonds are subjected to intense pressure deep inside the earth, the lattice of atoms can twist and bend. This creates small imperfections that diffract light and add colour to the gem.
Why is Argyle so rich in pink diamonds?
All diamonds are found in pipe-shaped volcanoes or their eroded remains. These volcanoes have deep roots under the continents where the diamonds are found.The roots must be deep. If they’re shallow, the carbon that could become diamonds is instead in the form of graphite, which is not nearly as attractive on an engagement ring.
The story of Argyle Volcano begins about 1,800 million years ago, when the continental plate beneath the Kimberley crashed into the rest of WA to form the first supercontinent, Nuna. Five hundred million years later, Nuna broke apart again while Australia held together.But old wounds never fully heal. The seam between the Kimberley and the rest of the continent was ripped open when Nuna split, and the Argyle volcano shot to the surface, bringing pink diamonds with it. The death of a supercontinent gave birth to Argyle.
So what made Argyle’s diamonds pink? The force that damaged the deep diamonds, giving them their beautiful colour, probably came from the continental collision that created the supercontinent in the first place. But the diamonds remained deep beneath this old wound for a long time before being brought to the surface.Will we find another pink diamond mine? With Argyle now closed, the search is on to meet the demand for these illustrious gems.The ingredients seem to be continental break-up, the edges of ancient continents and volcanic pipes.
Is carbon being recycled inside the Earth?
The search for diamonds is not just a search for glitz and glamour. It’s an exploration of the Earth’s deepest history.Diamonds are ancient time capsules from the depths of our planet. They are relics of a past so distant it defies comprehension.We know they are made of pure carbon – but where did that carbon come from?
Most of the carbon is left over from carbon-rich asteroids that clumped together to form the Earth 4.5 billion years ago.But some diamonds contain carbon that was once part of living organisms. Organic carbon from organisms that once thrived on the surface of the Earth has been buried deep underground by geological processes.
The Argyle diamonds, for example, contain such organic imprints, like echoes of an ancient world long gone. In these glimmers of the distant past, we find more than beauty; we find keys to unlocking the deepest secrets of our planet’s history.
