Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate):Limestone is an important stone for masonry and architecture, vying with only granite and sandstone to be the most commonly used architectural stone. Limestone is a key ingredient of quicklime, mortar, cement, and concrete. The solubility groundwater sources (ponds and streams), as surface water easily drains downward through cracks in the limestone. of limestone in water and weak acid solutions leads to important phenomena.