Diamonds are a geologist's best friend. Trapped inside the hardest natural substance is a relatively pristine record of the history of the continents — a record that can be exploited to investigate whether material that once formed the ocean floor of the early Earth was subducted into the mantle and preserved beneath the continental landmasses.

Earlier support for this view came from mantle rocks known as eclogite xenoliths. These are fragments of green, basalt-like material that were transported to the surface in volcanic magma. The ratio of the oxygen isotopes 18O and 16O in the eclogites suggested that they had originated as altered basalt on the ocean floor. But the hot, turbulent past of the xenoliths means that there is no guarantee that this geological record is pure.

In this issue (Nature 423, 68–70; 2003), Daniel Schulze and colleagues report their analysis of diamonds mined in Guaniamo, Venezuela. This image, captured through cathodoluminescence, shows one of their samples, 2 mm across and known as 'Picasso's diamond' for its resemblance to the cubist masterpieces of the Spanish painter.

Trapped inside this diamond (and others like it) is coesite, a form of silicon dioxide. The oxygen-isotope ratio in the coesite matches that of altered ocean-floor basalt — “compelling evidence”, say Schulze et al., in favour of subduction of oceanic plates being instrumental in the formation of the early continents.