sedimentary
Limestone (Chalk / Calcite Rock)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: Light beige, buff, cream, or off-white; Luster: Earthy, dull; Structure: Fine-grained microcrystalline or biogenic mass; Cleavage: Perfect rhombohedral in pure calcite crystals, but non-visible/massive in rock form; Specific Gravity: ~2.5–2.7.
- Hardness
- 3 on Mohs scale
- Color
- Light beige, buff, cream, or off-white
- Luster
- Earthy, dull
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Physical properties
Hardness: 3 on Mohs scale; Color: Light beige, buff, cream, or off-white; Luster: Earthy, dull; Structure: Fine-grained microcrystalline or biogenic mass; Cleavage: Perfect rhombohedral in pure calcite crystals, but non-visible/massive in rock form; Specific Gravity: ~2.5–2.7.
Formation & geological history
Formed in warm, shallow marine environments through the accumulation and compaction of microscopic calcium carbonate shells, skeletal debris, and chemical precipitation of calcite. Occurs worldwide across various geological ages from Precambrian to recent periods.
Uses & applications
Extensively used in construction, cement and lime production, soil acidity neutralizer in agriculture, aggregate for road bases, flux in steelmaking, and as a filler in paper, plastics, and paint.
Geological facts
Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Acid test (dilute HCl) causes it to effervesce vigorously, releasing carbon dioxide gas.
Field identification & locations
Easily identified by its soft nature (can be scratched with a steel nail) and strong effervescence with household vinegar or dilute hydrochloric acid. Commonly found in karst terrains, quarries, and coastal cliffs. Collectors look for unique marine fossils preserved within.
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