Diamonds from continental garbage.

Gem collectors reject diamonds marred by little black inclusions of foreign minerals. To some scientists, however, these flaws hold more value than the treasured rocks of romance themselves. New studies of diamond inclusions are now revealing how plate-tectonic activity pushes pieces of the continents into the interior of the planet.

Diamonds get their start deep in Earth's mantle, where pressures are high enough to force carbon atoms into a more densely packed crystal structure. The stones reach the surface when explosive volcanic eruptions propel them upward. Scientists originally thought diamonds grew from carbon supplied by the mantle. But in recent years they have found signs that one class of diamonds contains material from the ocean crust, which sinks down into the mantle at subduction zones around the world.

C. Stewart Eldridge of the Australian National University in Canberra and his colleagues now report that diamonds of this class also contain material from continental rocks. The evidence comes from an extremely sensitive machine called an ion microprobe, which the researchers used to analyze the ratios of sulfur and lead isotopes in the diamond inclusions.

The isotopic evidence provides clues to how the motion of Earth's tectonic plates mixes material from one layer of the planet into another. The researchers suggest that sediments essentially the refuse washed off the continents - get pulled into the mantle through subduction. Because the continental material becomes incorporated into diamonds, it must reach the significant depths where the gems grow, Eldridge says.