these are Metamorphic-Argillaceous rocks, which has
developed a well-marked splittable cleavages with or without
developed recrystallization & spots/knots of minerals like
garnet, pyrite, andulasite, etc. They are normally fine to
medium grained, moderately hard & compact, less homogeneous
with varying textures & shades. It has an ability to split
into thin layers along the plane of cleavages due to
flattened & elongated minerals along the plane of cleavage.