Xenolith
It happens most in igneous rock when magma is fluid enough to flow round more solid rock. Xenoliths may be covered in the margins of a magma chamber, torn loose from the walls of a flow of lava  or picked up along the ground by 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 low-silica lava and diamonds in kimberlite diatremes.