Sedimentary rocks form when particles of sediment, plant and animal matter are compressed and cemented together with minerals. The sedimentary rock that is formed mostly by deposits of shell fragments is limestone. Limestone can be an organic sedimentary rock that forms when shell fragments and sea fossils accumulate and are compacted or a biochemically produced rock formed from evaporation of seawater. Different kinds of limestone contain varying amounts of shell fragments and sea fossils
Fossil limestone is a coarse, rough rock packed with fossil fragments of many different sizes along with carbonate mud, carbonate cement or a combination of both. It is very soft when first uncovered, making it easy to work into architectural designs for columns or artistic sculptures before it hardens. It dissolves easily in acidic groundwater. Fossil limestone is light coloured and heavily patterned with the designs left by deposits of shell fragments.
Tennessee marble is a pink, red, grey or off-white limestone formed from tiny sea creatures such as bryozoans and crinoids. Although not genuine marble, which is a metamorphic rock, it is called marble because it's easily polished. Tennessee marble is a hard, smooth grade of limestone, which makes it useful in building and paving roads. According to Tennessee History for Kids, most of the marble in the U.S. Capitol building is Tennessee marble.
Coquina, or cockleshell, is a type of limestone formed from millions of fragments of shells from such creatures as clams, mollusks, coral, oysters and crinoids. Shell fragments combine with clay, calcite, phosphate, sand and fossils. Many coquina rocks have indentations from holes where trees used to grow. They vary in colour depending on the type of sand and shells that form them. Coquina from Florida has an orange colour from the red sand that combines with the shell fragments. The orange-tinted coquina can be found from Palm Beach extending northward into North Carolina.
Chalk is a type of marine limestone made entirely from marine microplankton. A soft, fine-textured limestone that is usually a white or pale grey colour, it breaks easily and dissolves in acidic solutions. Chalk is used to make putty, cement, lime and fertiliser. It is finely ground and used as a filler or pigment in materials such as cosmetics, plastic, rubber, crayons, paint, linoleum and ceramics. Chalk used for chalkboards, however, is usually made from a manufactured material that doesn't contain limestone.