sedimentary

Limestone

Calcium Carbonate (CaCO3 - Calcite)

Hardness: 3 on Mohs scale; Color: Gray to dark gray; Luster: Dull to earthy; Texture: Fine-grained, microcrystalline to sub-crystalline; Cleavage: Perfect rhombohedral in calcite grains, but massive in rock form

Hardness: 3 on Mohs scale
Color: Gray to dark gray
Luster: Dull to earthy

Identified July 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in shallow, warm marine environments from the accumulation and compaction of organic debris such as shell fragments, coral, and micro-algae, as well as chemical precipitation of calcium carbonate from seawater.

Uses & applications

Extensively used in construction as aggregate, in cement and lime manufacturing, soil conditioning in agriculture, industrial acid neutralization, and building stone.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. It readily reacts with dilute hydrochloric acid, producing visible effervescence (fissing/bubbling), which is a key diagnostic field test.

Field identification & locations

Identify by its gray color, softness (can be scratched by a steel nail), dull luster, and positive acid test. Common worldwide in sedimentary basins, roadcuts, and limestone quarries.