sedimentary
Limestone River Cobble
Sedimentary Calcium Carbonate (CaCO3)
Hardness: 3-4 (Mohs scale); Color: Gray to dark gray with white calcite veining; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None (conchoidal fracture)
- Hardness
- 3-4 (Mohs scale)
- Color
- Gray to dark gray with white calcite veining
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Gray to dark gray with white calcite veining; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the accumulation of calcium carbonate (shells, coral, and debris) in marine environments. This specific specimen has been rounded by mechanical erosion in a river or stream environment and features secondary calcite-filled fractures.
Uses & applications
Used extensively in the aggregate industry for road base, concrete, and as a source of lime for agriculture and steel manufacturing. Commonly collected as decorative river rocks.
Geological facts
Limestone accounts for about 10% of the total volume of all sedimentary rocks. It is essential for the carbon cycle and often contains microfossils that help geologists date the surrounding strata.
Field identification & locations
Identify by testing with a drop of vinegar or diluted HCl; it will effervesce (fizz). Look for thin white calcite veins and smooth, water-worn surfaces typical of river transport. Commonly found in riverbeds and mountainous terrain containing prehistoric seabeds.
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