sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Variable but frequently white, gray, or tan; Luster: Dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.71

Hardness: 3 on Mohs scale
Color: Variable but frequently white, gray, or tan
Luster: Dull to earthy

Identified December 25, 2025More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Variable but frequently white, gray, or tan; Luster: Dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.71

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in warm, shallow marine waters. It can also form through chemical precipitation from lake or ocean water. Most significant deposits range from the Paleozoic to Cenozoic eras.

Uses & applications

Extensively used in construction as building blocks, road base, and a key ingredient in cement/concrete. Used in agriculture to neutralize soil acidity and in the steel industry as a flux to remove impurities.

Geological facts

Limestone makes up about 10 percent of the total volume of all sedimentary rocks on Earth. It is the primary rock type found in karst landscapes, leading to the formation of massive cave systems through dissolution by slightly acidic groundwater.

Field identification & locations

Identify in the field by its reaction to cold dilute hydrochloric acid (it will fizz/effervesce). Look for fossils or stratified layering. It is commonly found in coastal regions, mountain ranges that were once ancient sea beds, and areas with extensive cave systems like those in Kentucky, USA, or the Mediterranean coast.