(Related: "Oldest Rocks on Earth Discovered?" [2008].)

Because Earth recycles its building blocks so thoroughly—particularly in the mantle, the layer just below the planet's relatively thin outer crust—no purely primordial material has been found before (overview of Earth's layers).

"We've been looking for pieces of the mantle that might have survived the chaotic mixing and churning within the deep Earth," said Boston University geochemist Matthew Jackson.

"But until now we haven't seen anything that looks like the original stuff that Earth formed from," added Jackson, who co-authored the study, published today in the journal Nature.

Even better, the newly analyzed rocks—which reached the surface some 62 million years ago—suggest a whole reservoir of the primordial rock could still lie somewhere