crystalline rock

crystalline rock

Any rock composed entirely of crystallized minerals without glassy matter (matter without visible crystals). Intrusive igneous rocks (see intrusive rock) are nearly always crystalline; extrusive igneous rocks (see extrusive rock) may be partly to entirely glassy. Metamorphic rocks are also always completely crystalline and are termed crystalline schists or gneisses. Sedimentary rocks can also be crystalline, such as crystalline limestones that precipitate directly from solution; the term is not generally applied to clastic sediments (made of fragments of preexisting rock), even though they are formed largely from the accumulation of crystalline materials.