Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, shallow marine waters where skeletal fragments and chemical precipitates accumulate over long spans of geologic time. For limestone to form, there must be an ample supply of calcium and carbonate ions in solution. https://logcla.com/blogs/485944/Limestone-The-Calcium-carbonate-rock-with-many-uses