Limestone is a biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. There are numerous varieties of limestone (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). The most distinctive type can be given the generalized name fossiliferous limestone, which is composed of abundant whole & fragmented fossil shells and skeletons. Most limestones represent deposition in ancient warm, shallow ocean environments.
Fossiliferous limestone from the Upper Ordovician (Kope Formation, lower Cincinnatian Series) of southwestern Ohio, USA. It is richly fossiliferous - mostly brachiopod shells, some bryozoans, some crinoid stem pieces, and some trilobite fragments.