In the last 20 years scientists have discovered new sources of diamond. Continental collisions -- a result of plate tectonics -- can subject slices of a crust to immense burial and uplift. In Kazakhstan, for example, diamonds formed in buried crust that returned to Earth's surface. Meteor impacts produce immense pressures, and diamonds can be formed and sprayed among the impact debris.

Meteorites also experience impacts themselves and can contain diamonds. And the most ancient meteorite material contains star dust, the remnants of the death of stars. Some of this star dust is extremely tiny bits of diamond, just big enough to be crystals and older than the solar system itself.

Very small "microdiamonds," averaging only 12 micrometers across, were discovered during diamond exploration in a region called the Kokchetav Massif, in northern Kazakhstan, in large slices of metamorphic rock that must have been pushed at least 120 kilometers deep into Earth and returned. Discovery of this process, termed ultrahigh pressure (UHP) metamorphism, has revolutionized ideas about and interest in what can happen to Earth's crust. Recently scientists have found traces of diamond around meteor impacts.

At the 35-million-year-old Popigai crater in Siberia, graphite transformed into microdiamond aggregates up to 1 centimeter across. It is now suspected that diamonds form in most major impacts, becoming a new indicator of ancient cosmic collisions. In 1987, microscopically small fragments of diamond, called "nanodiamonds," were recovered from meteorites that predate the solar system. New studies indicate that they formed more than 5 billion years ago in flashes of radiation from dying red-giant stars into surrounding clouds of methane-rich gas. The process is essentially the same as the new process for growing synthetic diamond called CVD -- chemical vapor deposition.

The diagrams above illustrate the formation of a UHP terrane that can yield diamonds. At top, the down-going subducted ocean crust (green) has a thin covering of sediment (gray) that is sheared off and driven upward (inset), apparently caused by the continental collision (middle) that squeezes the diamond-bearing metamorphic rocks back into the crust (bottom).