sedimentary
Coquina
Biogenic Limestone (Calcite/Aragonite, CaCO3)
Hardness: 2-3 Mohs; Color: Tan, white, or light gray; Luster: Dull or earthy; Crystal Structure: Amorphous fossil fragments; Cleavage: None; Specific Gravity: 2.2-2.7 (often lower due to porosity).
- Hardness
- 2-3 Mohs
- Color
- Tan, white, or light gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 2-3 Mohs; Color: Tan, white, or light gray; Luster: Dull or earthy; Crystal Structure: Amorphous fossil fragments; Cleavage: None; Specific Gravity: 2.2-2.7 (often lower due to porosity).
Formation & geological history
Formed in high-energy marine shoreline environments where wave action accumulates shell fragments. Over time, these fragments are naturally cemented together by calcium carbonate. Most deposits are relatively young (Pleistocene to Holocene).
Uses & applications
Historically used as a building stone (notably in Spanish forts); also used in road construction, landscape decoration, and as a source of lime for agriculture.
Geological facts
Castillo de San Marcos in St. Augustine, Florida, was built from coquina; its porous nature allowed it to absorb cannonball impacts rather than shattering. The name is Spanish for 'little shell' or 'cockle'.
Field identification & locations
Identify by its 'oatmeal-like' appearance and visible shell fragments. It will vigorously effervesce (fizz) when exposed to weak hydrochloric acid. Commonly found along the Atlantic coast from Florida to North Carolina.
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