Phyllite is a foliated metamorphic rock that has been subjected to low levels of heat, pressure and chemical activity. It is composed mainly of flake-shaped mica minerals in parallel alignment. The strong parallel alignment of the mica grains allows the rock to be easily split into sheets or slabs. The alignment of the mica grains gives phyllite a reflective sheen that distinguishes it from slate, its metamorphic precursor or protolith. Phyllite is usually gray, black, or greenish in color and often weathers to a tan or brown. Its reflective sheen often gives it a silvery, nonmetallic appearance.

Phyllite is a very common metamorphic rock, found in many parts of the world. It forms when sedimentary rocks are buried and mildly altered by the heat and directed pressure of regional metamorphism. These are almost always convergent plate boundary environments involving continental lithosphere.