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 crystalline forms); Specific Gravity: 2.5–2.7.

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

Identified March 12, 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 chemically through the precipitation of calcium carbonate from lake or ocean water. Ages can range from Precambrian to Holocene.

Uses & applications

Extensively used as an aggregate for road bases and railroad ballast. It is a primary ingredient in Portland cement and lime for soil treatment. Pure varieties are used in chemicals, glass making, and as a decorative building stone.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Most of the world’s most spectacular cave systems, like Mammoth Cave, are carved out of limestone by acidic groundwater.

Field identification & locations

Identify in the field by its 'fizz' reaction when exposed to dilute hydrochloric acid (acid test). Often contains visible fossils and exhibits a soft, scratchable surface. Found worldwide, particularly in areas that were once prehistoric seabeds.