sedimentary

Limestone or Chalk

Calcium Carbonate (CaCO3)

Hardness: 3 (Mohs scale), Color: White to light grey, Luster: Dull or earthy, Crystal structure: Hexagonal (as calcite), Cleavage: Perfect rhombohedral in calcite crystals but often invisible in fine-grained specimens, Specific gravity: 2.7. It is highly reactive to dilute hydrochloric acid.

Identified June 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also be formed by the chemical precipitation of calcium carbonate from lake or ocean water. Often found in Phanerozoic era strata.

Uses & applications

Extensively used in construction as an aggregate, cement production, agriculture (soil lime), and industrial chemical manufacturing. Finer varieties are used in paper, paint, and plastic as fillers.

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 are carved out of limestone beds due to its solubility in rainwater.

Field identification & locations

Identify in the field by testing for a vigorous effervescent reaction when exposed to a drop of white vinegar or dilute acid. It is commonly found in roadcuts and coastal cliffs in many parts of the world. Collectors look for fossils embedded within the matrix.