It takes a few things to make a diamond. First, you need carbon. Then you need a bunch of pressure and heat deep below the crust. And time – up to billions of years of it – for nature to do its job and then cough them up somewhere close to the planet's surface.
That's regular diamonds. If you want the rare pink kind a few more factors are required, not all of which are completely understood.
A study led by researchers from Curtin University in Australia has now provided an important missing piece of the puzzle explaining how these uniquely stunning gems make their way to the surface.
In addition to the pressure of colliding tectonic plates, pink diamonds require continents to stretch as they break apart. It's this taffy-like tugging that dredges the pink diamonds up from the depths of the mantle, where they can be discovered by humans scratching about topside.
Before closing in 2020, Argyle mine in the remote East Kimberley region of Western Australia supplied 90 percent of the world's pink diamonds. Discovering a critical step in their journey means we might be able to locate other crustal deposits of the rare gems.
"As long as these three ingredients are present – deep carbon, continental collision and then stretching – then we think it will be possible to find the 'next Argyle', which was once the world's largest source of natural diamonds," explains geologist Hugo Olierook of Curtin University in Australia.
While diamonds in general aren't especially rare, pink diamonds are among the rarest gems in the world. We don't know what makes them pink, either. Yellow or blue diamonds are colored by the presence of other elements, but pink diamonds are as chemically pure as the white variety.
What scientists do know is that they seem to require the immense forces involved in the collision of tectonic plates, deep in Earth's mantle. To figure out what else is needed to find pink diamonds, Olierook and his colleagues used geochronological techniques on rocks from the mine to identify the age of the deposits.
They looked at ratios of uranium in apatite and zircon that were part of the rocks in which pink diamonds are found. These rocks contain very, very small amounts of uranium and sometimes thorium, which decay over time into lead and helium. By measuring the ratio of uranium and its decay products in these minerals, scientists can obtain a very accurate age for the formation of the rock.
"By using laser beams smaller than the width of a human hair on rocks supplied by Rio Tinto," Olierook explains, "we found Argyle to be 1.3 billion years old, which is 100 million years older than previously thought, meaning it would likely have formed as a result of an ancient supercontinent breaking apart."
That supercontinent could be the ancient chunk of crust geologists call Nuna, which is estimated to have broken apart sometime between 1.3 and 1.22 billion years ago. The stretching of Nuna as it split might could be a vital component of the delivery of pink diamonds to the surface.
"Argyle is located at the point where the Kimberley region and the rest of northern Australia smashed together many years prior, and that sort of collision creates a damaged area or 'scar' in the land that will never fully heal," Olierook says.
"While the continent that would become Australia didn't break up, the area where Argyle is situated was stretched, including along the scar, which created gaps in the Earth's crust for magma to shoot up through to the surface, bringing with it pink diamonds."
This could help scientists figure out the formation process of pink diamonds, but also gives us clues about where to look for more.
However, it may not be easy. All of the world's diamonds can be found in volcanic regions. It seems that magmatic upwelling is required to transport the gems from deep underground.
This means, the researchers say, that most of them are found in the centers of ancient continents, where the volcanic regions are exposed on the surface.
"Argyle is at the suture of two of these ancient continents," Olierook explains, "and these edges are often covered by sand and soil, leaving the possibility that similar pink diamond-bearing volcanoes still sit undiscovered, including in Australia."
The research has been published in Nature.