The quadrillions of tons of diamond are buried in the "cratonic roots" of the earth's continents, and they are unfortunately inaccessible due to the fact that they're located around 100 miles underground.
"This shows that diamond is not perhaps this exotic mineral, but on the (geological) scale of things, it is relatively common", said Ulrich Faul, a research scientist in MIT. These formations - which are shaped like inverted mountains and lie at the center of the planet's tectonic plates, according to MIT News - stretch up to 200 miles into the Earth.
Accordingly, the scientists found that the sound waves velocities when passing through the cratonic roots were similar to those of the sound waves that passed through a mixture of rocks and up to 2% diamonds. The scientists discovered seemingly unpredictable changes in the speed of the waves as they traveled along, and the only explanation that seems plausible is that the waves are speeding up when they hit large deposits of diamond.
To do this, they first used seismic data to generate a 3D model of the velocities of seismic waves traveling through the Earth's major cratons. "Then we have to say, 'There is a problem.' That's how this project started". This might not seem much in terms of the total amount of material in the crust and mantle, but it is 1,000 times more diamonds than expected.
However, while there might be a treasure trove of fat, shiny diamonds underneath the earth's surface, chances are we won't be able to get our hands on them.
Parts of Earth's mantle may be up to two percent diamond by composition, far more than previously suspected.
Sound travels through diamond twice as fast as other rocks, so the team of researchers figured there had to be some of the material in the cratons. "Since these areas are hard to sample directly but are not impossible, because sometimes parts of these cratonic roots are brought to the surface by removing the magma, it is now the best explanation".
'Cratons are a tiny bit less dense than their surroundings, so they don't get subducted back into the Earth but stay floating on the surface. Down there, diamonds may not be so rare after all - it's just hard for them to make it to the surface and onto the rings and earrings of human beings.
Diamonds fetch their lofty price tags because they form over millions of years under high pressures and temperatures deep within the Earth's crust.
"It's circumstantial evidence, but we've pieced it all together", Faul said.
This research was supported, in part, by the National Science Foundation.