Limestone

Physical Properties

Hardness: 3 on Mohs scale; Color: white, tan, or gray; Luster: dull to earthy; Crystal structure: Trigonal (microcrystalline calcite); Cleavage: none in bulk rock form; Specific gravity: 2.5–2.7

Formation & Geological History

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through chemical precipitation of calcium carbonate from lake or ocean water.

Uses & Applications

Used extensively in construction as road base, aggregate for concrete, and as a building stone. In industry, it is used for making cement and as a soil conditioner (lime) in agriculture.

Geological Facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. It is the primary rock type found in massive cave systems and karst landscapes due to its solubility in acidic water.

Field Identification & Locations

Identify in the field by its softness (can be scratched by a steel nail) and its effervescence (fizzing) when in contact with dilute hydrochloric acid. Commonly found in areas that were once prehistoric seafloors.

Identified on: 4/21/2026

Mode: Standard