sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3-4 (Mohs scale); Color: Typically light grey to tan but can be dark grey; Luster: Dull to earthy; Crystal structure: Trigonal/rhombohedral (microcrystalline); Cleavage: Perfect rhombohedral in calcite, but usually not visible in massive field rock; Specific gravity: 2.3–2.7

Hardness: 3-4 (Mohs scale)
Color: Typically light grey to tan but can be dark grey
Luster: Dull to earthy

Identified March 17, 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. Most accessible limestone belongs to the Paleozoic or Mesozoic eras. Over millions of years, the calcium carbonate precipitates and cements organic remains into solid rock.

Uses & applications

Primary source of lime; widely used in construction as an aggregate for road bases and railroad ballast; used in the manufacture of cement, glass, and as a soil conditioner in agriculture to neutralize acid soils.

Geological facts

Limestone landscape features, known as 'Karst topography,' often contain extensive cave systems and sinkholes. About 10% of the total volume of all sedimentary rocks on Earth is limestone. It is also the primary material used in the construction of the Great Pyramids of Giza.

Field identification & locations

Identify in the field by its characteristic gray color, weathered pitted texture (vugs), and its reaction to dilute hydrochloric acid (it will effervesce or fizz). It is commonly found in areas that were once prehistoric seafloors, such as Central Texas, the Midwest US, and Southern England.