sedimentary
Coquina
Biogenic Limestone (Calcium Carbonate, CaCO3)
Hardness: 2-3 (Mohs scale); Color: tan, grey, or off-white; Luster: earthy or dull; Crystal Structure: contains visible shell fragments; Cleavage: none; Specific Gravity: varies (low due to porosity)
- Hardness
- 2-3 (Mohs scale)
- Color
- tan, grey, or off-white
- Luster
- earthy or dull
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Physical properties
Hardness: 2-3 (Mohs scale); Color: tan, grey, or off-white; Luster: earthy or dull; Crystal Structure: contains visible shell fragments; Cleavage: none; Specific Gravity: varies (low due to porosity)
Formation & geological history
Formed in high-energy marine environments where shell fragments and coral debris accumulate and are weakly cemented together by calcium carbonate. Most specimens date back to the Holocene or Pleistocene epochs.
Uses & applications
Historically used as a building material for forts and homes (especially in Florida), as road base material, and occasionally in landscaping or as a source of lime.
Geological facts
Coquina was used to build the Castillo de San Marcos in St. Augustine, Florida. Because it is porous, it absorbed cannonball impacts rather than shattering, making the fort nearly indestructible.
Field identification & locations
Identify it by its coarse, 'oatmeal-like' texture made of visible broken seashells. It reacts (fizzes) strongly when a drop of diluted hydrochloric acid or vinegar is applied. Common along coastal regions.
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