sedimentary
Coquina
Biogenic Limestone (Calcium Carbonate, CaCO3)
Hardness: 2-3 (Mohs scale); Color: Brown, tan, cream, or greyish-white; Luster: Dull or earthy; Structure: Fragmental/clastic composed of shell debris; Cleavage: None (friable/crumbly); Specific Gravity: 1.5 - 2.4.
- Hardness
- 2-3 (Mohs scale)
- Color
- Brown, tan, cream, or greyish-white
- Luster
- Dull or earthy
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Physical properties
Hardness: 2-3 (Mohs scale); Color: Brown, tan, cream, or greyish-white; Luster: Dull or earthy; Structure: Fragmental/clastic composed of shell debris; Cleavage: None (friable/crumbly); Specific Gravity: 1.5 - 2.4.
Formation & geological history
Formed in high-energy marine environments (beaches) where wave action accumulates broken shells, coral, and other organic debris. Over time, these fragments are naturally cemented together by calcium carbonate. Most deposits are relatively young (Pleistocene to Holocene).
Uses & applications
Used as a historical building material (e.g., Castillo de San Marcos), road base aggregate, landscaping stone, and in the production of lime and cement. It is also a popular specimen for educational fossil collecting.
Geological facts
Coquina is famous for its 'energy-absorbing' quality in military history; during the 17th century, cannonballs fired at coquina forts would sink into the soft walls rather than shattering them. It is the state rock of Florida.
Field identification & locations
Identify it by its distinctive 'oatmeal-like' appearance consisting almost entirely of small shell fragments. It is highly reactive to cold dilute hydrochloric acid (it will fizz). Commonly found along coastal areas in Florida, North Carolina, and Australia.
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