sedimentary

Coquina

Biogenic Limestone (Calcium Carbonate, CaCO3)

Hardness: 3 (Mohs scale); Color: tan, brownish-white, or gray; Luster: earthy/dull; Crystal Structure: trigonal (microscopic calcite crystals); Cleavage: none; Specific Gravity: 2.3-2.5. Highly porous and poorly cemented.

Hardness: 3 (Mohs scale)
Color: tan, brownish-white, or gray
Luster: earthy/dull

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in high-energy marine environments (beaches or shallow water) where wave action accumulates shell fragments and coral debris faster than they can dissolve. These fragments are loosely cemented by calcite over thousands of years.

Uses & applications

Used as a historical building material (especially in Florida and the Caribbean), road base substrate, landscaping rock, and a source of lime in agriculture. It is a popular specimen for educational geology kits.

Geological facts

Famous historical buildings like Castillo de San Marcos in St. Augustine, Florida, were built from coquina because its porous structure allows it to compress and absorb the impact of cannonballs rather than shattering.

Field identification & locations

Identify in the field by the visible presence of crushed shell fragments and a 'crunchy' texture. It will effervesce (fizz) aggressively when exposed to a drop of dilute hydrochloric acid (white vinegar also works). Found along fossiliferous coastline outcrops.

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