Jules Verne might say the rocks formed 20,000 leagues under the sea then journeyed to the center of the Earth.
Plate tectonics, movements of the Earth's crust caused by magma flowing underneath in the mantle, propelled the basalt downward into the lower mantle.
The basalt was then compressed into diamonds and returned to near the Earth's surface by magma plumes. The plumes hardened into diamond rich kimberlite deposits. The long-distance diamonds were then recovered by miners and studied by scientists.
"We looked at the variations in the isotopes of the carbon atoms in the diamonds," said study co-author Steven Shirey of the Carnegie Institution in a press release.
"Carbon originating in a rock called basalt, which forms from lava at the surface, is often different from that which originates in the mantle, in containing relatively less carbon-13. These super-deep diamonds contained much less carbon-13, which is most consistent with an origin in the organic component found in altered oceanic crust," said Shirey.