sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 (Mohs scale), Color: light gray, Luster: dull to earthy, Cleavage: none (conchoidal fracture), Specific Gravity: 2.5-2.7

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 3 (Mohs scale), Color: light gray, Luster: dull to earthy, Cleavage: none (conchoidal fracture), Specific Gravity: 2.5-2.7

Formation & geological history

Formed in marine environments from the accumulation of shell, coral, algal, and fecal debris; typically Paleozoic to Cenozoic in age.

Uses & applications

Used primarily in construction as road base, railroad ballast, and a key ingredient in portland cement; also used as an agricultural soil neutralizer.

Geological facts

Limestone covers about 10% of the Earth's total land surface and is the primary rock type found in karst landscapes, where it dissolves to form massive cave systems.

Field identification & locations

Identify by its light gray color and a positive reaction (fizzing) when exposed to a drop of weak hydrochloric acid (HCl); incredibly common in the Midwest and Southern US.