Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the surface. Xenoliths are geologists' only samples from the deep Earth, and carry information about diamond growth conditions. The 2 most common types of xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle beneath the crust and consists primarily of olivine -- the gem variety is peridot. Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle. Certain kinds of xenoliths contain diamonds.

These diagrams show the compositions of mantle xenoliths. Shown above is Lherzolite, a variety of peridotite thought to form most of the upper mantle.

Harzburgite is another kind of peridotite with less clinopyroxene. Garnet harzburgites contain red garnet and, occasionally, diamonds.

Eclogite, a very different rock, consists of garnet and sodium-rich pyroxene; some also contains diamonds.