sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3-4 (Mohs scale); Color: White, gray, tan; Luster: Dull to earthy; Crystal structure: Trigonal (microscopic); Cleavage: Perfect rhombohedral in calcite; Specific gravity: 2.5-2.7

Hardness: 3-4 (Mohs scale)
Color: White, gray, tan
Luster: Dull to earthy

Identified May 1, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale); Color: White, gray, tan; Luster: Dull to earthy; Crystal structure: Trigonal (microscopic); Cleavage: Perfect rhombohedral in calcite; Specific gravity: 2.5-2.7

Formation & geological history

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

Uses & applications

Used primarily in construction for building stone, aggregate for roads, and as a raw material for cement/lime. Also used in agriculture to neutralize soil acidity.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. Most of the world's caves are formed in limestone because it is soluble in weak acidic solutions like rainwater.

Field identification & locations

Can be identified by a 'fizz test' (effervescence) when a small amount of dilute hydrochloric acid or strong vinegar is applied. Commonly found in areas that were once prehistoric seafloors.