sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically gray, tan, or white; Luster: dull to earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific Gravity: 2.5–2.7.

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

Identified April 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through chemical precipitation from ocean water or lakes. Most accessible limestone is between 300 million to 500 million years old.

Uses & applications

Extensively used in construction as aggregate for roads, railroad ballast, and in the production of cement. It is also used as a soil conditioner and in the manufacturing of glass and steel.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary material found in famous cave systems like Mammoth Cave due to its solubility in slightly acidic water.

Field identification & locations

Identify in the field by its distinctive gray color and its reaction (fizzing) when exposed to a drop of dilute hydrochloric acid. It often contains visible fossils of marine organisms. Commonly found in quarries and cliff faces.