Diamond is not perhaps this exotic mineral, but on the (geological) scale of things, it is relatively common.
There may be more than a quadrillion tonnes of diamond hidden 100 miles below the surface of the Earth, far deeper than any drilling expedition has ever reached, according to a study.
Scientists from the Massachusetts Institute of Technology (MIT) in the US and colleagues said the findings are unlikely to set off a diamond rush.
The ultradeep cache may be scattered within cratonic roots - the oldest and most immovable sections of rock that lie beneath the centre of most continental tectonic plates, they said.
Shaped like inverted mountains, cratons can stretch as deep as 200 miles through the Earth's crust and into its mantle; geologists refer to their deepest sections as "roots."
In the study published in the journal Geochemistry, Geophysics, Geosystems, scientists estimate that cratonic roots may contain one to two per cent diamond.
Considering the total volume of cratonic roots in the Earth, the team figures that about a quadrillion tonnes of diamond are scattered within these ancient rocks, 90 to 150 miles below the surface.
"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.
"We can't get at them, but still, there is much more diamond there than we have ever thought before," Faul said.
The researchers came to the conclusion after puzzling over an anomaly in seismic data.
For the past few decades, agencies such as the US Geological Survey have kept global records of seismic activity - essentially, sound waves travelling through the Earth that are triggered by earthquakes, tsunamis, explosions, and other ground-shaking sources.
Seismic receivers around the world pick up sound waves from such sources, at various speeds and intensities, which seismologists can use to determine where, for example, an earthquake originated.
Scientists can also use this seismic data to construct an image of what the Earth's interior might look like.
Sound waves move at various speeds through the Earth, depending on the temperature, density, and composition of the rocks through which they travel.
Scientists have used this relationship between seismic velocity and rock composition to estimate the types of rocks that make up the Earth's crust and parts of the upper mantle, also known as the lithosphere.