Geologists Unravel the Mysteries of Australia’s Rare Pink Diamonds

During its operation, the Argyle mine in Western Australia was among the world’s largest sources of natural diamonds. But it has revealed more than just standard gemstones: It’s also responsible for more than 90 percent of the natural pink diamonds—ultra-rare gems with a reddish or blush-colored hue—that have ever been discovered.

Geologists have long puzzled over the mysterious origins of this prolific jewel-producing site. For one, it’s located near the edge of the continent—an unusual spot for diamonds, which typically emerge in the middle of continents. Argyle’s diamonds are also encased in a type of volcanic rock called lamproite, while most diamonds are found in kimberlite.

Beyond all that is the obvious: the diamonds’ unusual pink color. Diamonds form in Earth’s upper mantle under intense heat and pressure. They’re typically colorless, but if squeezed enough, they’ll turn pink—pressed even more, they’ll turn brown. The hue comes from the bending of their crystalline structure.

Now, scientists say they may finally have some answers as to how these rare diamonds came to be. After analyzing Argyle’s rocks and minerals with a laser beam smaller than the width of a human hair, a team of researchers has reached a new understanding of what happened at the site. They describe their findings in a new paper published Tuesday in the journal Nature Communications.

The story goes like this: Around 1.8 billion years ago, two sections of continental crust collided to form a supercontinent, which scientists already knew about and have named Nuna. The site of the continental crashing is where the Argyle diamond deposit sits today—and that event is what contorted the gems’ crystal structure and turned them pink.

Then, some 500 million years later—roughly 1.3 billion years ago—the supercontinent broke up. Nuna did not split at the exact location of Argyle, scientists say. But the forces of the breakup stretched the area. This weakened the region, allowing deep magma to erupt and bring the 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,” says study lead author Hugo Olierook, an Earth scientist at Australia’s Curtin University, in a statement.

The Argyle diamond mine opened in 1983 and operated for nearly 40 years before closing in 2020, when the remaining pink diamonds became too hard to access. But this new picture of the Argyle deposit’s origins suggests other pink diamond troves may exist elsewhere on the planet—they just haven’t been discovered yet. If they, too, formed near the edges of continents, they might be buried under sediment and rock, Olierook tells Live Science’s Stephanie Pappas.

“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,’” he adds in the statement.

The researchers measured radioactive decay in various minerals from Argyle to determine their age. Their findings push back the estimate for when the pink diamonds first emerged, dating Argyle to 100 million years earlier than previously thought. Shortly after the cache’s discovery, scientists had studied the diamonds and concluded they rose to the surface around 1.2 billion years ago.

But they weren’t totally convinced by that estimate, in part because they suspected the rocks they analyzed had been altered by fluids in the Earth. Beyond that, they were skeptical because “nothing was really happening [geologically] in Australia at that time,” as Olierook tells Science News’ Nikk Ogasa. The new timeline seems to be a better fit, he tells the publication, since it aligns with the breakup of Nuna.

Still, the new findings don’t answer all the questions swirling around Argyle. For instance, scientists still wonder why the formation contained so much carbon to begin with, since diamonds are made almost entirely of carbon.

“This is the last part of the story,” says Steve Shirey, a geochemist at the Carnegie Institution for Science who was not involved in the research, to the New York Times’ Maya Wei-Haas.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.