sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Tan, beige, or light gray; Luster: Dull to earthy; Crystal Structure: Trigonal (often microcrystalline); Cleavage: Perfect rhombohedral (but rarely seen in bulk rock); Specific Gravity: 2.5 - 2.7

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

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in warm, shallow marine waters. Can also form through chemical precipitation from calcium-rich water. Age varies from Precambrian to Holocene.

Uses & applications

Widely used in the construction industry as aggregate for road base and railroad ballast, in the production of cement and lime, as a soil conditioner in agriculture, and as a dimension stone for architecture.

Geological facts

Limestone covers about 10% of the total surface area of the Earth. It is the primary component of many famous landscapes, including the White Cliffs of Dover and the pyramids of Giza, which were encased in Tura limestone.

Field identification & locations

Identify in the field by the 'acid test'—it will vigorously effervesce (fizz) when a drop of dilute hydrochloric acid is applied. Commonly found in areas that were once prehistoric seafloors. Collectors look for fossil inclusions.