sedimentary

Coquina

Biogenic Limestone (Calcite, CaCO3)

Hardness: 3 (Mohs scale); Color: Tan, beige, or white with orange iron staining; Luster: Dull/Earthy; Crystal Structure: Amorphous/Granular; Specific Gravity: 1.5 - 2.5

Hardness: 3 (Mohs scale)
Color: Tan, beige, or white with orange iron staining
Luster: Dull/Earthy

Identified March 17, 2026More sedimentary →

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Physical properties

Hardness: 3 (Mohs scale); Color: Tan, beige, or white with orange iron staining; Luster: Dull/Earthy; Crystal Structure: Amorphous/Granular; Specific Gravity: 1.5 - 2.5

Formation & geological history

Formed in high-energy marine environments (shores and beaches) where wave action accumulates shells and shell fragments. The specimen is likely from the Pleistocene or Holocene epoch, cemented together by calcium carbonate.

Uses & applications

Historically used as a building material (notably in Spanish forts like Castillo de San Marcos), road base material, and in landscaping or aquarium decor.

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 cannonball fire during historical battles rather than shattering.

Field identification & locations

Identify in the field by its 'oatmeal' or 'popcorn' appearance consisting entirely of visible shell fragments. Common in coastal regions like Florida, North Carolina, and Australia. It will fizz when exposed to dilute hydrochloric acid.