Xenolith

n. mineral which is foreign to the rock in which it is located

This article concerns the geologic term, for other uses see: Xenolith (disambiguation) A xenolith (Greek: 'foreign rock') is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing lava on Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes.
See more at Wikipedia.org...

Noun
1. (geology) a piece of rock of different origin from the igneous rock in which it is embedded
(hypernym) rock, stone
(classification) geology

n. xenolith, mineral which is foreign to the rock in which it is located

A preexisting rock embedded in a newer igneous rock. Xenoliths are formed when a rising magma incorporates the preexisting rock. If the preexisting rock does not melt, it will not be assimilated into the magma and will therefore remain distinct from the new igneous rock that surrounds it.