Its main mineral constituent is quartz, and it is known to be very hard and abrasive. Another common clastic rock, conglomerate, looks like a mixture of different-sized rounded pebbles cemented together. Conglomerate forms when rapidly moving water drops pebbles into sand at the bottom of a river or stream. Over time, the pebbles and sand become cemented together. The most common chemical sedimentary rock is limestone.
Much limestone is made up of marine animals and contains pieces of shells, corals, and mollusks.
Another chemical sedimentary rock is coal, which is often found in layers with other sedimentary rocks. As plant remains are deposited in layers and slowly alter into carbon over millions of years, they form coal. Sedimentary rocks often preserve some of the features of the depositional process. For example, little waves or ripple marks may develop in the sand on a beach. The rock that forms from that sand may have rippled lines.