sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: White to light grey or tan; Luster: Dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral in calcite crystals; Specific Gravity: 2.5-2.7.

Hardness: 3 on Mohs scale
Color: White to light grey or tan
Luster: Dull to earthy

Identified June 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form chemically through the precipitation of calcium carbonate from lake or ocean water.

Uses & applications

Extensively used as a building material, in the manufacturing of cement, as an aggregate for the base of roads, and as a white pigment or filler in products such as toothpaste or paints.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. Many of the world's most famous caves, like Mammoth Cave in Kentucky, are formed by the dissolution of limestone by acidic groundwater.

Field identification & locations

Identified in the field by its softness (can be scratched by a penny) and its strong effervescence (fizzing) when in contact with a drop of dilute hydrochloric acid. Commonly found in areas that were once ancient sea beds.