sedimentary
Coquina
Bioclastic Limestone (composed primarily of CaCO3)
Hardness: 2-3 (Mohs scale); Color: White, tan, or cream; Luster: Dull/Earthy; Structure: Clastic/Biochemical (poorly cemented shell fragments); Cleavage: None (friable/crumbly); Specific Gravity: 2.3-2.6
- Hardness
- 2-3 (Mohs scale)
- Color
- White, tan, or cream
- Luster
- Dull/Earthy
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Physical properties
Hardness: 2-3 (Mohs scale); Color: White, tan, or cream; Luster: Dull/Earthy; Structure: Clastic/Biochemical (poorly cemented shell fragments); Cleavage: None (friable/crumbly); Specific Gravity: 2.3-2.6
Formation & geological history
Formed in high-energy marine environments (beaches or shallow reefs) where mechanical wave action accumulates shell fragments. Most existing coquina deposits are from the Pleistocene or Holocene epochs.
Uses & applications
Historically used as a building stone (e.g., Castillo de San Marcos); currently used in road base, landscaping, and the manufacture of cement.
Geological facts
Coquina is unique because it is porous and compressible, which allowed it to absorb cannonballs during colonial battles rather than shattering like solid stone. It is essentially a 'rock made of shells.'
Field identification & locations
Identify in the field by its 'granulized' appearance made of visible shell fragments and its vigorous effervescence when exposed to dilute HCl. It is commonly found along the Atlantic coast, particularly in Florida.
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