Evan Smith wanted to get his hands on the world's biggest diamonds — the kind that sit atop royal scepters, and the ones that are always the target of elaborate movie heists.

But this wasn't for some nefarious get-rich-quick scheme. It was for science.

"The most valuable, the most prized, of all gemstones are coincidentally some of the most scientifically valuable pieces of the Earth," says Smith, a diamond geologist at the Gemological Institute of America.

They're scientifically valuable because they come from a deep part of the Earth that humans can't access and don't know that much about.

Because of their rare size and quality, Smith thought these diamonds might have come from somewhere different, though no one knew exactly where.

"It was a total mystery," says Smith.

To solve that mystery, he'd have to look inside the diamonds, at tiny specs of junk no wider than a human hair that the crystals had brought with them on their journey from the deep.

"You really couldn't ask for a better vessel to store something in. Diamond is the ultimate Tupperware," says Smith.

A slogan like "the ultimate Tupperware" won't sell many engagement rings, but for scientists, the diamonds' Tupperware-quality is key. It makes the geologic equivalent of messages in a bottle.

But Smith couldn't just knock on a royal palace door and ask to crack open the crown jewels.

Instead, he got the Gemological Institute of America to buy eight fingernail-sized chunks of those big diamonds, the scraps leftover from when the rough diamonds were cut into sparkly jewels.

After grinding some down and cutting others open, Smith used fancy techniques involving big microscopes, lasers and electron beams to figure what was inside. He also used some not-so-fancy equipment — a magnet attached to a string — to figure out if they contained iron. ("After staring at these inclusions for hours on end over the course of many months, you start to resort to some alternative tools," he says).

Smith eventually found that many of the stones contained bits of garnet with a silicon content indicating that they must have formed under very high pressure. He also found iron and nickel, shrouded in invisible envelopes of fluid methane.

"That's unusual. This is the first time I've seen methane around an inclusion," he says.

When he took a nondestructive look at 53 other diamonds passing through the institute for quality grading, he found that 38 of them contained the same unusual materials.

As Smith and his colleagues wrote Thursday in the journal Science, those odd bits and pieces told him two important things.

"One, they tell us that these large, exceptional-quality diamonds originate from extreme depths in the Earth," he says, from about 200 to 500 miles below us.

That's about as far under our feet as the International Space Station is above our heads. And it's about twice as deep as where most diamonds are born.

"So, that in itself is pretty amazing," says Smith.

The second thing he learned is that the diamonds had formed inside oxygen-deprived patches of liquid metal. And that's the first hard evidence that the Earth's mantle is not a uniform stew of oxygen-rich rocks.

It might not sound very exciting, says Kanani Lee, a mineral physicist at Yale University, but it is.

"It further complicates things, but it makes us have to think more deeply about what's going on in the planet because ultimately this does affect what we see up on the surface," says Lee.

As the Earth cooled over the last 4.5 billion years, its layers slowly revolved from the core to the surface and back again. Until recently, scientists expected that the mantle, the part of the planet between the continental plates and its core, would be pretty thoroughly mixed, with oxygen distributed throughout. But these diamonds show that until relatively recently, there were pockets that somehow managed to resist that mixing.

And those pockets were long-lasting and widespread enough to produce diamonds that surfaced on multiple continents and that range in age from about 100 million years old to about a billion years old.

It's unclear if those pockets are still around now. Nevertheless, it means that the planet and its past could be a little messier than scientists first thought.

"It tells you that we have to refine our thinking about how the planet – whether it's Earth or any other planet — evolves with time. And that our simple pictures may not be good enough anymore if we can't explain these features," says Lee.

Those odd features are just slivers of a much larger picture — how Earth became what it is today, including its ability to host life.

"Over time, those are the things that shape the surface of the Earth. They're the materials that the whole surface of the Earth is built with," Smith says.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

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