sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Gray, tan, white, or buff; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: Perfect rhombohedral (rarely visible in hand samples); Specific gravity: 2.5–2.7.

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

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in marine environments through 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. Geological age ranges from Precambrian to Holocene.

Uses & applications

Extensively used in construction as an aggregate for road bases and railroad ballast, a component in cement/concrete, a soil conditioner in agriculture, and for architectural facing stone.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. Most of the world's largest cave systems are formed in limestone because it is soluble in slightly acidic groundwater.

Field identification & locations

Identify in the field by its characteristic gray color and fine-grained texture; it will effervesce (fizz) vigorously when a drop of 10% hydrochloric acid is applied. It is commonly found in areas that were once ancient sea beds.