sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on the Mohs scale; Color: typically white, gray, or tan; Luster: dull to earthy; Crystal structure: hexagonal/trigonal (if crystalline calcite is present); Cleavage: perfect rhombohedral in calcite crystals, though usually massive in rock form; Specific gravity: 2.5 - 2.8.

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

Identified March 20, 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 through chemical precipitation from ocean or lake water. Formation ages range from the Precambrian to Holocene.

Uses & applications

Extensively used as a building material, in the manufacturing of cement (Portland cement), as an aggregate for the base of roads, as a white pigment or filler in products like toothpaste or paints, and as a soil conditioner (agricultural lime).

Geological facts

Limestone is the source of many of the world's most famous caves, as it is easily dissolved by acidic groundwater. It often contains abundant fossils of marine life. Most of the Great Pyramids of Giza were originally clad in smooth, white Tura limestone.

Field identification & locations

Identify in the field by its softness (can be scratched by a copper coin or steel knife) and its reaction to dilute hydrochloric acid, which will cause it to effervesce (fizz). It is commonly found in large bedded deposits in regions that were once undersea, such as the Midwest United States, Western Europe, and coastal areas.