Organic sedimentary rocks are sedimentary rocks formed from the remains of organisms. The three types of organic sedimentary rock are limestone, chert and coal. Both limestone and chert are formed largely from the hard structures of organisms, while coal is formed from dead plants after a long period of exposure to the correct environmental conditions.
Bony fish, corals and certain planktonic algae form calcium-based structures that break down and settle to the ocean floor after their deaths. These build up under time and pressure, eventually solidifying into a solid mass known as limestone. Limestone is easily dissolved by acidic substances, even including rainwater, because of its calcium content. Marble is a metamorphic rock made from limestone exposed to extreme temperatures and pressures.
Another group of photosynthetic plankton, known as diatoms, as well as sponges create structures from silica. The sediments from diatoms are the source of diatomaceous earth, but when not gathered for this purpose time and pressure cause the formation of chert.
Coal is the only type of organic sedimentary rock that is not formed primarily in oceans. Instead, it form in shallow waters, particularly swamps, where plant debris is buried by sediment and exposed to various chemical processes, purifying the carbon component of the plants.