Stratus (St) Found at very low levels (less than 2000 meters, or 6500 feet), stratus clouds are thick, large, heavy-looking gray clouds that dominate the sky. Precipitation normally does not occur with this cloud, although drizzle or mist may be observed.
Shale is a fine-grained sedimentary rock which is formed by the compaction of silt and clay-size mineral particles. Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism.
Other rocks are much finer grained than granite, so it's not easy to spot the different minerals. Slate is a rock that was made from clay, and clay is composed of tiny, tiny particles. Those particles can be minerals like quartz, pyrite, apatite, muscovite, feldspar, kaolinite, biotite, tourmaline - the list goes on!