sedimentary
Coquina Limestone
Biogenic Limestone (Calcium Carbonate, CaCO3)
Hardness: 3 (Mohs scale); Color: Cream, tan, or grayish-white; Luster: Dull or earthy; Structure: Fragmental/Clastic composed of shells; Cleavage: None (friable/crumbly); Specific gravity: 2.3 to 2.5.
- Hardness
- 3 (Mohs scale)
- Color
- Cream, tan, or grayish-white
- Luster
- Dull or earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Cream, tan, or grayish-white; Luster: Dull or earthy; Structure: Fragmental/Clastic composed of shells; Cleavage: None (friable/crumbly); Specific gravity: 2.3 to 2.5.
Formation & geological history
Formed in high-energy marine environments (beaches or shallow reefs) where shell fragments accumulate and are naturally cemented by calcite. Most common in Holocene and Pleistocene deposits.
Uses & applications
Historically used for building fortifications (it absorbs cannonballs without shattering) and decorative architecture. Today used as road base material and landscaping stone.
Geological facts
Famous for being the primary building material of the Castillo de San Marcos in Florida, which resisted British attacks for decades because the soft coquina absorbed projectiles.
Field identification & locations
Identify in the field by looking for visible fossil shell fragments or whole shells held together with minimal cement. Found in coastal areas like Florida or North Carolina. It will effervesce (fizz) under weak acid like vinegar.
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