Science

Bizarre 'nesting doll' diamond found inside another diamond

The first of its kind yet known, the unusual gem from a Russian mine hints at mysterious machinations at work miles beneath our feet.Friday, 11 October 2019

By Maya Wei-Haas
In a first, a newfound diamond freely rattles inside the belly of a bigger diamond. Scientists are puzzling over how such an unusual configuration came to be.

While sorting through the gems freshly unearthed in Yakutia, Russia, specialists working with the mining company ALROSA stumbled across something they’d never seen before: a tiny diamond entombed in the belly of a larger diamond. The so-called matryoshka diamond, named after Russian nesting dolls, weighs a mere 0.124 grams, and the outer gem is about as wide as a grain of rice is long, according to ALROSA’s announcement about the find last week.

Diamonds frequently act like tiny time capsules, capturing exotic minerals or traces of the soupy liquid that birthed them. But the discovery of an itty-bitty diamond rattling around inside a natural case of more diamond has stunned researchers.

“I thought, Holy cow, I never saw anything like that,” says University of Alberta minerologist Thomas Stachel of his reaction after watching video of one diamond jingling inside the other. “We have been looking for diamonds now for a long time, and that is a first.”

This x-ray image of a diamond duo shows one tiny gem nestled within an empty cavity of a larger diamond. Such a void cannot exist under the crushing pressures deep inside our planet where diamonds form, so researchers think something must have once filled the cavity.

It’s unclear exactly how the diamonds acquired their curious configuration, but clues may soon be on the horizon: Plans are in the works to send the tiny sparklers to the Gemological Institute of America for further study, which could help bring to light new details about the machinations at work miles beneath your feet.

“This is really a unique creation of nature,” Oleg Kovalchuk, deputy director for innovations at ALROSA's Research and Development Geological Enterprise, says in a press release.

Diamond roots

Diamonds frequently crystallise some hundred or so miles below Earth’s surface. They take shape in what’s known as the cratonic roots of continents, which are zones of ancient, stiff mantle that bouy the overlying landmasses. (Learn more about the estimated quadrillion tons of diamonds that lurk deep inside Earth.)

These diamond-baking regions lie well out of reach of human exploration; the deepest anyone’s ever drilled is little more than 7.6 miles down. So to study the curious conditions of their formation, scientists rely on analysing the diamonds themselves, which are brought to the surface during rare volcanic eruptions that dredge up deep molten rocks known as kimberlite magma.

A smaller diamond moves within a larger one in an animated image. Researchers at the Gemological Institute of America hope to study the bizarre gem for clues about its formation.

That’s likely how this new diamond came within reach of ogling humans—but how exactly it formed remains a mystery. The crystallisation of many diamonds seem to be tied to seafloor subduction, which occurs when a dense oceanic tectonic plate plunges beneath a less dense continental plate. As the seafloor descends, temperatures rise, forcing fluids from the rocks and sediments. Diamonds crystallise from the resulting salty, carbon-rich soup that percolates through the subsurface. (Read about how Earth’s seafloor may be destined to become diamonds.)

But as the newfound diamond was crystallising, it seems, something went awry. Instead of the gem forming as one single hunk of mineral, the tiny jewel was enveloped inside a bigger diamond. The currently empty cavity in which the tiny diamond now resides could not have existed under the crushing pressures the duo would have encountered deep underground, so something else must have initially filled the void.

“You can’t have open space in the mantle. That’s totally and utterly impossible,” Stachel says. “Any open space at these pressures would disappear in a millisecond.”

Mystery filling

Perhaps the double diamond could have once housed a drop of the salty fluid in which the mineral took shape, says Michael Förster, a postdoctoral researcher in experimental petrology at Australia's Macquarie University. Such a fluid could have readily leaked out through a hole or crack in the larger diamond casing. Other experts suggest that the space was once filled with minerals from the mantle, such as brown-green olivine or dark red garnet.

“It is interesting to imagine what mineral could have worked as a spacer between the inner and outer diamond,” Förster writes in an email.

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It’s unclear how and when these minerals would have then disappeared, but they could have oozed away when the diamonds were en route to the surface, says Wuyi Wang, vice president of research and development at the Gemological Institute of America. Molten rock or searing fluid could have come in contact with the filling and liquified the mantle minerals.

Alternatively, Stachel thinks the transformation could have happened at Earth’s surface. There, water could have worked away at the mineral filling, first transforming it into weaker, softer minerals and then perhaps dissolving some of them away.

Yet it’s unlikely that either process cleaned out the entire cavity. That’s where human processing probably came in, Stachel emphasises. While it’s unclear how the double diamond was cleaned after being mined, many processing methods include a wash with strongly corrosive substances, like hydrofluoric acid.

“That dissolves almost any mineral that we know,” Stachel says. One of the few that survives is diamond. Requests for more information from ALROSA about the diamond and its processing are still pending.

Dazzling future

Previously, other curious diamond structures have been found that seem to echo parts of the new diamond-in-diamond structure. For one, Wang has long owned what seems to be a diamond inside a diamond but the inner gem still remains firmly attached to the inner wall of its diamond case. Also, many other diamonds have holes punched into their dazzling surface, Stachel adds.

By studying these oddballs of the mineral world, researchers can learn more about the process of how diamonds grow, and perhaps the environments and chemistries the stones encounter deep inside our planet.

Wang, who will lead the analysis of the gem at GIA, is particularly interested in examining the double diamond with a high resolution CT scan to visualise the structure in three dimensions. He hopes to examine the shape of the inner cavity, as well as search for a possible escape route for the probable mineral filling. He also hopes to study the diamonds’ chemistries non-destructively for clues to what may have once lingered within the empty space.

Yet to truly solve the problem, Stachel says, they just may need to find more examples: “The ultimate goal would be that these guys now find a diamond where the hole is still filled. Then we’d all be very happy.”

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