Limestone is a geological rock formed primarily from calcium carbonate and in water. It is produced principally during periods when there is a high proportion of carbon dioxide in the atmosphere. It varies greatly in its hardness and in the materials included in it so its commercial uses vary. Limestone, a common sedimentary rock composed primarily of the mineral calcite. Limestone constitutes approximately 10 percent of the sedimentary rocks exposed on the earth’s surface. Limestone is very common in architecture, especially in Europe and North America.