sedimentary
Coquina
Bioclastic Limestone (composed primarily of CaCO3)
Hardness: 3 (Mohs scale); Color: Light tan, white, or cream; Luster: Dull/Earthy; Crystal Structure: Orthic/Hexagonal (calcite); Cleavage: Perfect rhombohedral in calcite grains; Specific Gravity: 2.7.
- Hardness
- 3 (Mohs scale)
- Color
- Light tan, white, or cream
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Light tan, white, or cream; Luster: Dull/Earthy; Crystal Structure: Orthic/Hexagonal (calcite); Cleavage: Perfect rhombohedral in calcite grains; Specific Gravity: 2.7.
Formation & geological history
Formed in high-energy marine environments (beaches) where wave action breaks shells and coral into fragments. These are later accumulated and cemented by calcite through biochemical precipitation. Most deposits are relatively young (Pleistocene).
Uses & applications
Used as building stone (notably in historic forts like Castillo de San Marcos), road ballast, landscape mulch, and a source of lime for agriculture.
Geological facts
Coquina is unique because it is soft when underwater but hardens when exposed to air; historically, it was an effective defense material as it would absorb cannonballs rather than shattering like harder stone.
Field identification & locations
Identify in the field by its distinct 'oatmeal' or 'breakfast cereal' appearance consisting entirely of visible shell fragments. Found primarily on coastal regions, especially Florida and North Carolina.
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