sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Gray, tan, or white; Luster: Dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral in calcite particles; Specific Gravity: 2.5–2.73

Hardness: 3 on Mohs scale
Color: Gray, tan, or white
Luster: Dull to earthy

Identified February 15, 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 lake or ocean waters. Deposits can span from the Precambrian to Holocene eras.

Uses & applications

Used widely in construction (aggregate, cement manufacture), agriculture (soil conditioner), and as a building stone. High-purity limestone is used in glass making and chemical processes.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. Most of the world’s spectacular caves, such as Mammoth Cave in Kentucky, are formed by the dissolution of limestone by slightly acidic groundwater.

Field identification & locations

Identify in the field using the 'acid test'—it will effervesce (fizz) when a drop of 10% hydrochloric acid or vinegar is applied. It is commonly found in ancient seabed regions, road cuts, and quarries. Collectors look for fossiliferous specimens containing shells or crinoids.