sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically gray, off-white, or tan; 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 gray, off-white, or tan
Luster: dull to earthy

Identified March 29, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: typically gray, off-white, or tan; Luster: dull to earthy; Crystal structure: trigonal (as calcite); Cleavage: perfect rhombohedral; Specific gravity: 2.5–2.7

Formation & geological history

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through chemical precipitation from lake or ocean waters. Found in geological formations ranging from the Precambrian to the Holocene.

Uses & applications

Used extensively as a building material (cut stone), aggregate for road bases, a key ingredient in cement, and as a soil conditioner (agricultural lime) to neutralize acidity.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Because it is soluble in weak organic acids, it is the primary rock type in which vast cave systems and karst landscapes are formed.

Field identification & locations

Identify in the field by applying a drop of dilute hydrochloric acid (or strong vinegar); it will effervesce (fizz) vigorously. Look for fossils or a fine-grained, non-crystalline surface. It is extremely common worldwide, especially in the Midwestern United States and Europe.