Additional heat, pressure, and chemical activity might convert the schist into a granular metamorphic rock known as "gneiss." Garnet mica schist: This rock is composed of fine-grained muscovite mica with numerous visible grains of red garnet. The specimen shown is about two inches five centimeters across..Schists are cl.ified and named on basis of their mineralogy: The most common minerals of schist are mica; mica-schist contains quartz and mica bio.e or muscovite as main minerals, schist is often named after additional minerals, e.g..Schist is a medium-grade metamorphic rock. Schist has medium to large, flat, sheet-like grains Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual .