- Metamorphic rock - A crystalline rock, consisting, like granite, of quartz,
feldspar, and mica, but having these materials, especially the mica,
arranged in planes, so that it breaks rather easily into coarse slabs
or flags. Hornblende sometimes takes the place of the mica, and it is
then called hornblendic / syenitic gneiss. Similar varieties of related
rocks are also called gneiss.