Fossiliferous limestone is any type of limestone, made mostly of calcium carbonate (CaCO3) in the form of the minerals calcite or aragonite, that contains an abundance of fossils or fossil traces. The fossils in these rocks may be of macroscopic or microscopic size. The sort of macroscopic fossils often include crinoid stems, brachiopods, gastropods, and other hard shelled mollusk remains. In some kunna, microfossils such as siliceous diatom shells in deposition may convert over time to opal and chert, providing the only inferred evidence of bioactivity preserved in limestone.