sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically white, gray, or tan; Luster: earthy to dull; Crystal structure: Orthorhombic or Trigonal (as calcite/aragonite); Cleavage: perfect rhombohedral in crystal form; Specific Gravity: 2.5-2.7

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

Identified April 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 through chemical precipitation from calcium-rich water in caves or mineral springs.

Uses & applications

Extensively used in construction as aggregate, road base, and as a key ingredient in cement (Portland cement). It is also used in agriculture for soil neutralization and in the steel manufacturing process.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Most of the world's great cave systems, such as Mammoth Cave, are carved out of limestone by acidic groundwater.

Field identification & locations

Identify in the field by its softness (can be scratched by a copper coin or steel knife) and its strong effervescent reaction ('fizzing') when a drop of diluted hydrochloric acid or vinegar is applied. Often contains visible fossils.