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.