sedimentary

Gray Limestone Pebble

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Gray with white calcite veining; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: Perfect rhombohedral in calcite particles

Hardness: 3 on Mohs scale
Color: Gray with white calcite veining
Luster: Dull to earthy

Identified June 25, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Gray with white calcite veining; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: Perfect rhombohedral in calcite particles

Formation & geological history

Formed from the accumulation of marine skeletal fragments (coral, foraminifera, and mollusks) or chemical precipitation in marine environments. This specimen appears water-worn from a river or beach environment.

Uses & applications

Used in the production of cement, as a soil conditioner in agriculture, a building material, and in the production of lime.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Most of the world's caves are formed in limestone beds by the dissolution of calcium carbonate by acidic groundwater.

Field identification & locations

Identify by a 'fizz' test using diluted hydrochloric acid or vinegar (reacts with calcium carbonate). Commonly found in ancient sea beds, riverbanks, and coastal areas. Collectors look for fossils or unique veining.