sedimentary

Coquina

Coquina (Bioclastic Limestone)

Hardness: 2-3 (Mohs scale); Color: Tan, cream, white, or light brown; Luster: Earthy to dull; Structure: Clastic/Bioclastic, composed of fragmented shells; Cleavage: None; Specific Gravity: 2.3-2.5

Hardness: 2-3 (Mohs scale)
Color: Tan, cream, white, or light brown
Luster: Earthy to dull

Identified April 11, 2026More sedimentary →

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Physical properties

Hardness: 2-3 (Mohs scale); Color: Tan, cream, white, or light brown; Luster: Earthy to dull; Structure: Clastic/Bioclastic, composed of fragmented shells; Cleavage: None; Specific Gravity: 2.3-2.5

Formation & geological history

Formed in high-energy marine environments (beaches or shallow reefs) where wave action breaks down shells and mollusks into fragments, which are then cemented together by calcite over thousands of years.

Uses & applications

Used as a historical building material (notably in Spanish forts like Castillo de San Marcos), road base material, decorative landscaping, and occasionally as an abrasive.

Geological facts

Coquina was an ideal material for coastal forts because it is relatively soft and compressible; rather than shattering when hit by a cannonball, the porous rock would simply absorb the impact like 'geological memory foam'.

Field identification & locations

Identify by its 'oatmeal-like' appearance and clearly visible shell fragments. In the field, look for it in coastal deposits. It will react and fizz strongly when exposed to a drop of muriatic or hydrochloric acid.