Wherever it came to rest, this roiling proto-rock brought plenty of souvenirs along for the ride, samples from down deep and samples from every crash and bend along the way. When the magma cooled, these fragments were encased, captured forever as a record of the places left behind. As such, they represent a veritable window into the mantle.

The most exotic of kimberlite's inclusions are diamonds.

Diamonds form at depths of over 150 kilometers, down where pressures are high enough to compress carbon into its densest form. Kimberlite magma carried these rare crystals up to the surface, where they were first stumbled on in streambeds.

Then in 1868, diamonds were discovered on farmlands in a remote part of Cape Province, South Africa, far from any river's banks. In the ensuing scramble, these gems were traced to pipes of blue rock -- diatremes. The material and the mining town that sprang up were later named after the British Colonial Foreign Secretary of the time, Lord Kimberly.

Diamonds appear very rarely even in kimberlite; when found, only the tiniest fraction are valuable. "A really rich sample," says Duff Gold, Penn State professor of geology, "would contain about 100 carats of diamond per 100 tons of rock." A carat, he adds, weighs a fifth of a gram.