Recycled crust hails a glittering taxi.

The ride dragged on for millions of years. But when the passenger finally arrived, it was in high style, ensconced in the heart of a diamond.

This first-class voyager was a speck of continental crust that traveled deep into the fiery innards of Earth and then made a return trip to the surface inside a South African diamond. The discovery of this unusual occupant provides the best evidence yet that continental rock gets recycled into the planet's interior, according to a report in the Jan. 11 Nature by Leon R.M.

Daniels of the University of Cape Town in South Africa and his coworkers.

Geophysicists know that Earth's dense oceanic crust can sink into the mantle through a process called subduction. They have long suspected that subduction also carries pieces of the continents into the mantle. Until now, though, they have lacked definitive evidence.

Diamonds form in the mantle at depths of more than 120 kilometers, where intense pressure squeezes carbon atoms into their most compact form. Mantle minerals often get trapped inside the growing gemstones. When volcanic eruptions carry those diamonds to the surface, they include the mantle rocks in the ride upward.

Daniels' group, however, found an inclusion that apparently did not come from the mantle. The researchers identified a speck of the mineral staurolite in a South African diamond. Staurolite is a common metamorphic mineral in continental crust, but no one had ever seen it before in mantle rock. "It seems most likely that this staurolite crystal formed within metamorphosed crustal rocks, which were subsequently carried into the mantle through subduction," comments William M. White of Cornell University in the same issue of Nature.

If a substantial amount of continental rock has been subducted over the eons, the process will have limited the continental area existing today.