Xenoliths
Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's layer to the surface. Xenoliths are geologists' only samples from the deep Earth, and bring information about diamond growth conditions. The 2 most general 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 mainly of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle. Those kinds of xenoliths contain diamonds.
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