sedimentary

Limestone

Calcareous Sedimentary Rock (CaCO3)

Hardness: 3 on Mohs scale; Color: typically gray, white, or tan; Luster: dull to earthy; Crystal structure: hexagonal/rhombohedral (microcrystalline calcite); Cleavage: none (conchoidal to irregular fracture); Specific gravity: 2.5–2.7

Hardness: 3 on Mohs scale
Color: typically gray, white, or tan
Luster: dull to earthy

Identified March 7, 2026More sedimentary →

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Physical properties

Formation & geological history

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

Uses & applications

Extensively used in construction as an aggregate for road bases and concrete, in the manufacture of lime and cement, as a soil conditioner in agriculture, and for building stone and sculpture.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary source of carbon dioxide for the atmosphere when it subducts or undergoes metamorphism, and it is the rock type that defines karst topography, including caves and sinkholes.

Field identification & locations

Can be identified in the field by its distinctive 'fizz' or effervescence when exposed to a drop of dilute hydrochloric acid (HCl). It is commonly found in areas that were once prehistoric seafloors. Collectors look for fossiliferous limestone containing visible ancient marine life.