Limestone

Physical Properties

Hardness: 3 on Mohs scale; Color: Gray, tan, white, or buff; Luster: Dull to earthy; Crystal Structure: Hexagonal/Trigonal (in calcite form); Cleavage: Perfect rhombohedral; Specific Gravity: 2.5 - 2.7.

Formation & Geological History

Formed primarily through 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. Geological age varies widely from Precambrian to Holocene.

Uses & Applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. Also used in the production of cement, for soil neutralization in agriculture, and as a filler in products like toothpaste and paint.

Geological Facts

Limestone contains about 10% of the total volume of all sedimentary rocks. Most of the world's caves are formed in limestone when acidic groundwater dissolves the rock over millions of years. It often contains a rich record of ancient fossils.

Field Identification & Locations

Identify in the field by its characteristic gray color and fine-grained texture; it will fizz ('effervesce') vigorously when a drop of 10% hydrochloric acid is applied. Commonly found in areas that were once covered by ancient seas.

Identified on: 4/23/2026

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