Not all diamonds are created equal - the most sought-after gems are both extraordinarily clear and large, and for decades, scientists have been trying to figure out what forces must come together to form the most valuable diamonds on Earth.
Now a new analysis of the imperfections trapped within these stones has given us the answer - the biggest and most valuable diamonds are formed inside isolated pools of liquid metal hundreds of kilometres below Earth's surface.
"The most valuable, the most prized, of all gemstones are coincidentally some of the most scientifically valuable pieces of the Earth," one of the team, Evan Smith, a diamond geologist from the Gemological Institute of America, told NPR.
It's common knowledge that diamonds form deep below the surface of the planet, in the hot, dense rock of Earth's mantle, between the core and the crust. We eventually gain access to them when they're brought to the surface by volcanic eruptions.
But it's not that simple, because unusually large and clear diamonds appear to be fundamentally different from all the rest - it's not that they're just a better version of the same thing, they appear to differ from less valuable diamonds on both a structural and chemical level.
The most famous example of this is the Cullinan diamond - the biggest gem-quality diamond ever found. Before it was cut into smaller pieces, it weighed 621.35 grams (1.37 lbs) and stretched 9.8 cm long (3.86 inches) - and it's also incredibly clear.
"Some of the world's largest and most valuable diamonds, like the Cullinan or Lesotho Promise, exhibit a distinct set of physical characteristics that have led many to regard them as separate from other, more common diamonds," explains one of the team, Wuyi Wang, in a press statement.
"However, exactly how these diamonds form and what they tell us about Earth has remained a mystery until now."
To figure this out, the team had to gain access to a number of type II diamonds - the rarest form of diamonds on Earth.
These diamonds are made from almost pure carbon, meaning they're usually clear, and there are barely any nitrogen impurities in the crystal lattice - inclusions that give the cheaper stones their yellowish hue.
Type IIa diamonds make up 1 to 2 percent of all natural diamonds, and the even more unblemished type IIb diamonds make up about 0.1 percent of all natural diamonds.
First, the researchers sourced nine offcuts of type II diamonds, and by grinding them down or cutting them open, were able to analyse the minute inclusions in the crystal structure.
They found that a few of the offcuts contained grains of majorite - a type of garnet found in Earth's upper mantle, that only forms under very high pressures.
That wasn't so surprising, we already know that diamonds require insane amounts of pressure to form, but then they found that other inclusions were made from a mixture of iron, nickel, carbon, and sulphur - a combination that's never been seen in a 'common' diamond before.
They also found traces of methane and hydrogen that had formed around these inclusions like a shroud - something that had also never been observed in a diamond before.
"That's unusual. This is the first time I've seen methane around an inclusion," Smith told Rae Ellen Bichell NPR.
Next, the team sourced 42 polished and processed type II diamonds, and overall, found that 72 percent of the total samples contained these strange iron and nickel alloys. Garnet inclusions were found in 15 of the 53 samples.
Based on these chemical signatures, the researchers were able to pinpoint how far down into Earth's mantle the stones were originally formed, and what conditions had to come together to achieve such unusual combinations.
The fact that they include garnet means they could be forming as deep as 750 kilometres (466 miles) below the surface - any deeper, and garnet becomes unstable. To put that into perspective, the International Space Station orbits at 350 km (220 miles) above Earth.
Even crazier is the fact that for the methane-enveloped inclusions to have survived the diamond-forming process, they would need to have crystallised within small, isolated pockets of metallic liquid inside the mantle - separate from where 'regular' diamonds form.
Common diamonds are known to form in a far more extensive, uniform 'stew' of oxygen-rich rocks in the mantle.
This suggests "that there is an ocean of liquid metal deep in Earth's mantle," Smith told Charles Q. Choi at Live Science, split into isolated pockets, "limited to perhaps fist-sized, if I were to guess, that are peppered throughout the mantle".
"There's not a lot of this metallic iron - just about 1 percent or so of the mantle," he adds.
It's not clear how much of this liquid metal there is in the mantle, or where it comes from, but the researchers are hoping to figure that out next by looking even deeper into their diamond samples.
"That might help shed light on the origin of this metal," says Smith. "Where does it come from, how does it form, what lifetime does it have, what processes does it participate in?"
The research has been published in Science.