sedimentary
Coquina
Bioclastic Limestone (composed primarily of CaCO3)
Hardness: 3-4 (Mohs scale); Color: Tan, brownish-yellow, or whitish-gray; Luster: Dull or earthy; Crystal Structure: Amorphous/Microcrystalline; Cleavage: None (friable); Specific Gravity: 2.2-2.5. Characterized by a coarse, 'cheerios' like texture made of visible shell fragments.
- Hardness
- 3-4 (Mohs scale)
- Color
- Tan, brownish-yellow, or whitish-gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Tan, brownish-yellow, or whitish-gray; Luster: Dull or earthy; Crystal Structure: Amorphous/Microcrystalline; Cleavage: None (friable); Specific Gravity: 2.2-2.5. Characterized by a coarse, 'cheerios' like texture made of visible shell fragments.
Formation & geological history
Formed in high-energy marine shoreline environments where wave action accumulates shell debris. Mechanical sorting of shells and coral fragments occurs, followed by light cementation by calcium carbonate. Most deposits are relatively young, dating from the Pleistocene to Holocene epochs.
Uses & applications
Used primarily in construction as a road base material, decorative stone for landscaping, and historically as a building stone (notable for its impact-absorbing properties in fortifications).
Geological facts
Coquina was used to build the Castillo de San Marcos in St. Augustine, Florida. It is unique as a building material because its porous nature allows it to absorb cannonball fire rather than shattering like harder stone.
Field identification & locations
Identify in the field by looking for a 'rock made of shells' appearance that is poorly cemented and easily crumbled with a tool. Commonly found along the Atlantic coast of Florida and North Carolina.
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