Limestone

Physical Properties

Hardness: 3 on Mohs scale; Color: white to off-white/tan; Luster: dull to earthy; Crystal structure: Trigonal (microcrystalline in this form); Cleavage: none (conchoidal to irregular fracture); Specific Gravity: 2.5-2.7

Formation & Geological History

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form chemically through the precipitation of calcium carbonate from lake or ocean water.

Uses & Applications

Dominantly used in construction as an aggregate for road bases and railroad ballast. Also used as a component in cement, for soil neutralization in agriculture, and as a feedstock in the glass industry.

Geological Facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary component of most cave systems because it is soluble in slightly acidic groundwater, leading to the formation of stalactites and stalagmites.

Field Identification & Locations

Identify in the field by its softness (can be scratched by a copper coin) and its vigorous reaction (fizzing) when a drop of dilute hydrochloric acid or strong vinegar is applied. Commonly found in areas that were once prehistoric seafloors.

Identified on: 4/26/2026

Mode: Standard