sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically gray, tan, or white, but often weathered to dark gray; Luster: dull to earthy; Crystal structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific gravity: 2.71.

Hardness: 3 on Mohs scale
Luster: dull to earthy

Identified February 19, 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 the chemical precipitation of calcium carbonate from lake or ocean water. Most common specimens date from the Paleozoic or Mesozoic eras.

Uses & applications

Extensively used in construction as building stone, road base, and a key ingredient in cement. Industrially, it is used for toothpaste, paper, plastics, and as a soil conditioner to neutralize acidity in agriculture.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary rock found in karst topography, leading to the formation of extensive cave systems and sinkholes due to its solubility in slightly acidic water.

Field identification & locations

Identify in the field by its tendency to fizz (effervesce) when in contact with dilute hydrochloric acid or strong vinegar. Look for a fine-grained texture and the presence of marine fossils. Common globally in areas that were once prehistoric sea beds.