Limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO3). It is formed when calcium carbonate precipitates out of water and accumulates on the seafloor. Limestone can be either coarse-grained or fine-grained, depending on the size of the calcite crystals that make up the rock. Coarse-grained limestone has larger calcite crystals, while fine-grained limestone has smaller calcite crystals. The grain size of limestone can vary depending on the environment in which it was formed. For example, limestone that is formed in a high-energy environment, such as a beach or a river, will typically be coarse-grained, while limestone that is formed in a low-energy environment, such as a lagoon or a lake, will typically be fine-grained.