A sedimentary rock composed mainly of calcium carbonate (CaCO₃), often with minor amounts of magnesium carbonate (MgCO₃), clay minerals, and other impurities. Different types of limestone exist depending on the mineral composition and grain size.

Limestone is formed through the accumulation and lithification (compaction and cementation) of the skeletal remains of marine organisms (corals, shells, etc.) over millions of years.