Mudstone is a fine-grained sedimentary rock formed from clays or muds.  It consists mostly of silicate minerals.  It differs from sandstone in having individual grains too small to see without a microscope.  Mudstone differs from shale by its lack of parallel layers.  Mudstone can be sufficiently silicified to be flaked into tools, and some varieties are suitable for heat-treatment.  It tends to have a slightly rough or coarse texture, and occurs in a variety of colours depending on the trace minerals.