sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Gray, tan, or white; Luster: Dull to earthy; Texture: Fine-grained to fragmental; Reaction: Fizzes strongly with dilute hydrochloric acid.

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

Identified July 19, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Gray, tan, or white; Luster: Dull to earthy; Texture: Fine-grained to fragmental; Reaction: Fizzes strongly with dilute hydrochloric acid.

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in clear, warm, shallow marine waters. Can also form through chemical precipitation from ocean water or lake water.

Uses & applications

Extensively used in construction as an aggregate for road bases and railroad ballast, a component in cement and mortar, and as a flux in steel manufacturing.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary material found in famous karst landscapes, including the White Cliffs of Dover and various world-famous cave systems.

Field identification & locations

Identify in the field by its softness (can be scratched by a copper coin) and its vigorous effervescence when exposed to acid. Found globally in areas that were once prehistoric seafloors.