Limestone

Hardness: 3 (Mohs scale); Color: White, cream, or light grey; Luster: Dull to earthy; Texture: Fine-grained to fragmental; Specific gravity: 2.5-2.7

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form through the precipitation of calcium carbonate from lake or ocean water. Most deposits span from the Paleozoic to Cenozoic eras.

Extensively used in construction as building stone, aggregate for concrete, and as a primary ingredient in cement. Also used in agriculture to neutralize soil acidity and as a raw material for the chemical industry.

Limestone makes up about 10% of the total volume of all sedimentary rocks. Many world-famous structures, such as the Great Pyramids of Giza and various European cathedrals, are built from limestone. It is also the primary rock type found in karst landscapes, leading to the formation of caves.

Can be identified in the field by its softness (readily scratched by a steel knife) and its strong effervescent reaction when exposed to cold dilute hydrochloric acid. It is commonly found in areas that were once prehistoric seafloors.