sedimentary

Limestone

Calcareous Sedimentary Rock (CaCO3)

Hardness: 3 on Mohs scale; Color: typically light gray, tan, or off-white; Luster: dull to earthy; Crystal structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral in calcite crystals; Specific gravity: 2.5–2.7.

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

Identified April 7, 2026More sedimentary →

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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 or lake water. Common across all geological ages, especially from the Paleozoic and Mesozoic eras.

Uses & applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. It is a key ingredient in cement, used as a soil conditioner in agriculture, and high-quality slabs are used for architectural cladding.

Geological facts

Limestone accounts for about 10% of the total volume of all sedimentary rocks. Many limestone formations contain abundant fossils, and large underground cave systems are typically formed by the dissolution of limestone by acidic groundwater.

Field identification & locations

Identify in the field by its reaction (fizzing) when exposed to a weak acid like white vinegar. It is commonly found in areas that were once prehistoric seafloors. Collectors look for specimens containing well-preserved marine fossils.