sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: Off-white, tan, or light yellow; Luster: Dull to earthy; Crystal Structure: Trigonal (as microcrystalline calcite); Cleavage: Perfect rhombohedral (rarely visible in hand samples); Specific Gravity: 2.5–2.7.

Hardness: 3 on Mohs scale (can be scratched by a copper coin)
Color: Off-white, tan, or light yellow
Luster: Dull to earthy

Identified March 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in shallow, warm marine environments through the accumulation of shell, coral, algal, and fecal debris. It can also form through chemical precipitation from calcium-rich waters. Geological age ranges from the Precambrian to the Cenozoic era.

Uses & applications

Extensively used in construction as an aggregate for road bases and concrete. Used in the manufacture of cement, as a flux in steel making, as a soil conditioner in agriculture, and as a raw material for the chemical industry. Higher-grade varieties are used for architectural building stone.

Geological facts

Limestone accounts for about 10% of the total volume of all sedimentary rocks. Most of the world's spectacular caves, such as Mammoth Cave in Kentucky, are formed by the dissolution of limestone by acidic groundwater.

Field identification & locations

Identify in the field by applying a drop of cold 10% hydrochloric acid (HCl); it will 'fizz' or effervesce vigorously. It is commonly found in ancient seabed deposits worldwide (e.g., Indiana, Florida, and the White Cliffs of Dover). Collectors look for specimens containing visible fossils like crinoids or ammonites.