sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Tan, buff, or light gray; Luster: Dull to earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None/Conchoidal; Specific Gravity: 2.3 to 2.7

Hardness: 3 on Mohs scale
Color: Tan, buff, or light gray
Luster: Dull to earthy

Identified June 29, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Tan, buff, or light gray; Luster: Dull to earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None/Conchoidal; Specific Gravity: 2.3 to 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 the precipitation of calcium carbonate from lake or ocean water. Most specimens date from the Phanerozoic Eon.

Uses & applications

Extensively used in construction as building stone, road base, and an ingredient in Portland cement. Used in agriculture to neutralize soil acidity and as a flux in steel making.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. Most caves are formed by the dissolution of limestone by slightly acidic groundwater.

Field identification & locations

Identified by its softness (can be scratched by a penny) and its strong effervescent reaction to dilute hydrochloric acid. Commonly found in areas that were once prehistoric sea beds.