sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically grey, tan, or white; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.5-2.7

Hardness: 3 on Mohs scale
Color: typically grey, tan, or white
Luster: dull to earthy

Identified April 4, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: typically grey, tan, or white; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.5-2.7

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form through chemical precipitation from ocean or lake water. Geological ages vary widely from Precambrian to Holocene.

Uses & applications

Used extensively in construction as road base, railroad ballast, and aggregate in concrete. It is the primary ingredient in cement and is used as a soil conditioner in agriculture to neutralize acidity.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary rock found in karst landscapes, leading to the formation of extensive cave systems and sinkholes.

Field identification & locations

Identify in the field by its readiness to fizz/effervesce when exposed to a drop of dilute hydrochloric acid. Commonly found in massive beds in ancient or modern seafloor locations. Collectors look for fossil inclusions.