sedimentary
Coquina
Bioclastic Limestone (composed primarily of Calcium Carbonate / CaCO3)
Hardness: 2-3 Mohs; Color: White, cream, tan, or grey; Luster: Dull to earthy; Structure: Granular and porous with visible shell fragments; Cleavage: None (conchoidal fracture when broken); Specific Gravity: 2.3-2.5.
- Hardness
- 2-3 Mohs
- Color
- White, cream, tan, or grey
- Luster
- Dull to earthy
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Physical properties
Hardness: 2-3 Mohs; Color: White, cream, tan, or grey; Luster: Dull to earthy; Structure: Granular and porous with visible shell fragments; Cleavage: None (conchoidal fracture when broken); Specific Gravity: 2.3-2.5.
Formation & geological history
Formed in high-energy marine environments (beaches or shallow reefs) where shells or coral fragments accumulate. Geologically, these range from the Pleistocene (recent) to older Cenozoic eras.
Uses & applications
Historically used as a building material for forts and coastal structures (notably St. Augustine, FL); also used as gravel, road base, or source of lime for agriculture.
Geological facts
Coquina is uniquely soft when first quarried but hardens upon exposure to air. This porosity allowed it to absorb cannonballs rather than shattering, making it an ideal fortification material.
Field identification & locations
Identify by looking for poorly cemented collections of broken clam shells or coral debris that react with dilute hydrochloric acid. It is commonly found along the Atlantic coast of Florida and North Carolina.
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