Xenolith
The word "xenolith" comes from the Greek xenos, which means "stranger" and lithos, meaning "stone."
These "stranger stones" are found around the margins of many igneous intrusions, where magma has melted and forced its way into other rocks. They may also be found in lava.
They are lumps of rock that have been broken off and engulfed by the igneous rock, so are strangers in their new location. Xenoliths often appear as dark, rounded, or irregular rocks set into granite or another igneous rock.
Near the very edge of the igneous intrusion, a xenolith will not have been heated too much by the magma and so keeps many of its original features. Xenoliths found some feet into the igneous rock will have been altered considerably and metamorphosed. They may even have crystals in them similar to those in the igneous rock.
Xenoliths help geologists to work out the types of rock the magma passed through, as it was being intruded or erupted.
In some places blocks of the very lowest crustal rocks have been brought to the surface by rising magma. These xenoliths allow geologists to study rocks not normally seen on the surface.
In the diamond-bearing rocks around Kimberly, South Africa, xenoliths that may be derived from the Earth's mantle (the region beneath the crust) are found.
Large amounts of xenolith rock caught up in magma may react with it and change its composition.