sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically grey, tan, or white; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral (when crystalline); Specific Gravity: 2.3 to 2.7.

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

Identified March 21, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form through chemical precipitation from ocean water or cave environments (speleothems). Found in geological ages ranging from Precambrian to Holocene.

Uses & applications

Extensively used in construction as aggregate, road base, and a component of cement. Used in agriculture to neutralize soil acidity, in the chemical industry for steel manufacturing, and occasionally as dimension stone for architecture.

Geological facts

Limestone covers about 10% of the Earth's land surface. Most of the world's most famous caves, such as Mammoth Cave in Kentucky, are carved out of limestone by acidic groundwater. It often contains abundant fossils of ancient marine life.

Field identification & locations

Identify in the field using a 'fizz test'—it reacts vigorously with dilute hydrochloric acid. It often has a gritty texture and may contain visible fossils. Common in areas with ancient sea beds like the Midwestern US, UK, and the Alps.