Limestone

Physical Properties

Hardness: 3 on Mohs scale; Color: Gray, light-gray to bluish-gray; Luster: Dull to earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: Perfect rhombohedral in calcite crystals but often not visible in rock form; Specific Gravity: 2.5-2.7.

Formation & Geological History

Formed primarily in shallow, warm marine waters from the accumulation of coral, shell, algal, and fecal debris. It can also form through chemical precipitation from calcium-rich water. It is found in geological layers ranging from the Precambrian to recent eras.

Uses & Applications

Major ingredient in cement and concrete; used as a building stone, road base, and rail ballast. In agriculture, it is used to neutralize soil acidity. High-purity limestone is used in glass making and various chemical industries.

Geological Facts

Limestone contains about 10% of the total volume of all sedimentary rocks. Most of the world's most famous caves, like Mammoth Cave in Kentucky, 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) and its strong effervescence (bubbling) when a drop of dilute hydrochloric acid or strong vinegar is applied. Widely distributed globally, notably in the Midwest USA, the Alps, and coastal regions.

Identified on: 4/22/2026

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