sedimentary

Coquina

Biogenic Limestone (Calcitic shells)

Hardness: 2-3 (Mohs scale); Color: Tan, cream, or light brown; Luster: Dull/Earthy; Crystal structure: Amorphous/Biogenic fragments; Cleavage: None (fragmental); Specific Gravity: 1.8-2.2

Hardness: 2-3 (Mohs scale)
Color: Tan, cream, or light brown
Luster: Dull/Earthy

Identified March 28, 2026More sedimentary →

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Physical properties

Hardness: 2-3 (Mohs scale); Color: Tan, cream, or light brown; Luster: Dull/Earthy; Crystal structure: Amorphous/Biogenic fragments; Cleavage: None (fragmental); Specific Gravity: 1.8-2.2

Formation & geological history

Formed in high-energy marine environments (beaches) where waves accumulate broken shell fragments and fossils. These fragments are later cemented together by calcium carbonate. Most deposits are relatively young (Pleistocene to Holocene).

Uses & applications

Historically used as a building stone (notably for forts due to its ability to absorb cannon fire), in landscaping, as road base material, and occasionally in jewelry as a novelty stone.

Geological facts

The Castillo de San Marcos in Florida was built of coquina. Because it is porous, the stone absorbed the impact of cannonballs rather than shattering like brick or granite.

Field identification & locations

Identify in the field by its 'oatmeal-like' appearance and visible shell fragments. It will effervesce (fizz) vigorously when touched with dilute hydrochloric acid or vinegar. Common in coastal areas of Florida, Australia, and Brazil.