sedimentary
Coquina / Shelly Limestone
Coquina (Biogenic Limestone)
Hardness: 3-4 (variable), Color: Cream, tan, or grayish-white, Luster: Dull/Earthy, Crystal Structure: Amorphous/Organic remains, Cleavage: None/Irregular, Specific Gravity: Low (highly porous)
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Physical properties
Hardness: 3-4 (variable), Color: Cream, tan, or grayish-white, Luster: Dull/Earthy, Crystal Structure: Amorphous/Organic remains, Cleavage: None/Irregular, Specific Gravity: Low (highly porous)
Formation & geological history
Formed in high-energy marine environments (beaches) during the Holocene or Pleistocene epochs. It consists of mechanically sorted shell fragments cemented together by calcium carbonate precipitation.
Uses & applications
Historically used as building stone (e.g., Castillo de San Marcos) due to its ability to absorb cannon fire. Currently used for decorative landscaping, road gravel, and concrete aggregate.
Geological facts
Coquina is unique because it is soft when first quarried but hardens when exposed to air. Its porous nature allowed it to absorb historical cannonballs rather than shattering like brick or granite.
Field identification & locations
Identify by looking for visible, broken shell fragments and a 'crunchy' texture. It will react and fizz strongly if a drop of vinegar (acid) is applied.
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