sedimentary

Coquina

Biogenic Limestone (Calcite, CaCO3)

Hardness: 2-3 (Mohs scale); Color: Tan, brown, or grayish-white; Luster: Dull or earthy; Structure: Fragmental, poorly cemented mass of entire or broken shells; Specific Gravity: 2.3–2.5.

Hardness: 2-3 (Mohs scale)
Color: Tan, brown, or grayish-white
Luster: Dull or earthy

Identified March 26, 2026More sedimentary →

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Physical properties

Hardness: 2-3 (Mohs scale); Color: Tan, brown, or grayish-white; Luster: Dull or earthy; Structure: Fragmental, poorly cemented mass of entire or broken shells; Specific Gravity: 2.3–2.5.

Formation & geological history

Formed in high-energy marine shoreline or near-shore environments where wave action accumulates large quantities of shell fragments. Most geologically young specimens date to the Pleistocene or Holocene epochs.

Uses & applications

Historically used as a building material for forts and homes (such as the Castillo de San Marcos in Florida) due to its ability to absorb cannonball impacts. Currently used for gravel, road base, landscaping, and as a lime source for agriculture.

Geological facts

Because it is porous and poorly cemented, coquina is a natural shock absorber. During the 1702 Siege of St. Augustine, Spanish defenders found that British cannonballs would sink into the coquina walls rather than shattering them.

Field identification & locations

Identify it by its 'oatmeal-like' appearance and visible shell/coral fragments. It will vigorously effervesce (fizz) when exposed to a 10% hydrochloric acid solution. Found commonly along the coasts of Florida, North Carolina, and Shark Bay, Australia.