sedimentary
Coquina / Shelly Limestone
Coquina (calcium carbonate, CaCO3)
Hardness: 2.5-3.5 on Mohs scale; Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Clastic with visible shell fragments and pores/cavities; Specific Gravity: 1.2 to 2.4 (often lightweight due to porosity).
- Hardness
- 2
- Color
- Gray, tan, or white
- Luster
- Dull to earthy
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Physical properties
Hardness: 2.5-3.5 on Mohs scale; Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Clastic with visible shell fragments and pores/cavities; Specific Gravity: 1.2 to 2.4 (often lightweight due to porosity).
Formation & geological history
Formed in high-energy marine environments (beaches or shallow reefs) where shells, coral, and mineral fragments accumulation is rapid. It is often from the Pleistocene or Holocene epochs but can be older.
Uses & applications
Historically used in construction (forts and buildings due to its ability to absorb cannon fire), road base material, aquarium landscaping, and as a source of lime for agriculture.
Geological facts
The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; the stone’s porous nature allowed it to absorb cannonballs rather than shattering, making the fort nearly invincible.
Field identification & locations
Identify by looking for visible shell fragments and an 'oatmeal' or 'brain-like' lumpy texture. It will react and fizz if a drop of vinegar (weak acid) is applied. Commonly found along the coasts of Florida and North Carolina.
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