Limestone (specifically Fossiliferous Limestone)

Physical Properties

Hardness: 3 on Mohs scale; Color: Gray to tan/brown; Luster: Dull or earthy; Crystal Structure: Trigonal (rhombohedral calcite); Cleavage: Perfect rhombohedral in calcite crystals, though often massive.

Formation & Geological History

Formed from the accumulation of calcium carbonate shells, corals, and skeletal fragments in warm, shallow marine environments. Over millions of years, pressure and chemical processes lithify these sediments into rock.

Uses & Applications

Primary source of lime and cement for construction; used as a building stone, a soil conditioner in agriculture, and in the production of iron in blast furnaces.

Geological Facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It often contains visible fossils, providing a record of prehistoric marine life from as far back as the Phanerozoic Eon.

Field Identification & Locations

Identify in the field by its tendency to effervesce (fizz) when exposed to a 10% solution of hydrochloric acid (or vinegar). Look for bedding planes and fossil inclusions. Common in areas that were once ancient seabeds.

Identified on: 4/22/2026

Mode: Standard