xenolith

This term is used exclusively within the specialized register of geology and petrology. It describes a physical inclusion that is chemically and mineralogically distinct from the surrounding host rock, serving as a geological "time capsule" that provides evidence of the composition of the Earth's crust or mantle from which the magma ascended.

Because it refers to a discrete physical object, it is treated as a standard countable noun. It is frequently paired with adjectives describing its origin or composition, such as "mantle xenolith" or "crustal xenolith," to specify the depth from which the fragment was plucked.