sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 (Mohs scale); Color: Varying shades of grey, tan, or brown; Luster: Dull to earthy; Crystal structure: Trigonal (though usually microcrystalline in hand specimens); Cleavage: Perfect rhombohedral (rarely visible in raw rock); Specific gravity: 2.3–2.7.

Hardness: 3 (Mohs scale)
Color: Varying shades of grey, tan, or brown
Luster: Dull to earthy

Identified March 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in shallow marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through the chemical precipitation of calcium carbonate from lake or ocean water. Most specimens date from the Phanerozoic eon.

Uses & applications

Extensively used in construction as an aggregate, a key ingredient in cement, and as a soil conditioner in agriculture. Pure varieties are used in pharmaceutical calcium supplements and industrial chemical manufacturing.

Geological facts

Limestone often contains fossils of marine organisms. Over time, under intense heat and pressure, limestone metamorphoses into marble. It is the primary component of stalactites and stalagmites found in caves.

Field identification & locations

Identify in the field by its reaction to dilute hydrochloric acid (it will fizz or effervesce). It is commonly found in areas that were once prehistoric seabeds. For collectors, it is best found in quarry sites or road cuts where fossiliferous layers are exposed.