Limestone

Physical Properties

Hardness: 3 on Mohs scale; Color: typically white, gray, or tan; Luster: dull to earthy; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral in calcite crystals; 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 through chemical precipitation from ocean or lake water. Geological ages can range from the Precambrian to Holocene.

Uses & Applications

Used extensively in construction (as gravel, cement, and building stone), in agriculture to neutralize soil acidity, as a flux in steel making, and in the production of glass and paper.

Geological Facts

Limestone is the source of approximately 10% of the total volume of all sedimentary rocks. It often contains abundant fossils that provide a record of ancient marine life. The Pyramids of Giza were largely constructed from limestone blocks.

Field Identification & Locations

Identify in the field by its softness (can be scratched by a copper coin or steel knife) and its reaction with dilute hydrochloric acid, which causes it to effervesce (fizz). Commonly found in areas that were once prehistoric seafloors.

Identified on: 5/13/2026

Mode: Standard