sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on the Mohs scale; Color: typically gray, white, or tan; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.71

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

Identified March 19, 2026More sedimentary →

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Physical properties

Hardness: 3 on the Mohs scale; Color: typically gray, white, or tan; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.71

Formation & geological history

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through the chemical precipitation of calcium carbonate from lake or ocean water. It can range in age from Precambrian to the present day.

Uses & applications

Extensively used in construction as an aggregate for road bases and railroad ballast. It is a key ingredient in cement (Portland cement) and lime. It is also used in agriculture to neutralize soil acidity and as a building stone for facades.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. It is the primary host rock for many of the world's most famous cave systems, such as Mammoth Cave, due to its solubility in slightly acidic rainwater.

Field identification & locations

Identify in the field by its gray color and the presence of fossils; the definitive test is using a weak acid (like vinegar or HCl), which will cause it to effervesce (fizz). Commonly found in areas that were once prehistoric seafloors.