Fossiliferous Limestone (Coquina / Shell Limestone)

Physical Properties

Hardness: 3 (Mohs scale); Color: Brown, tan, and grey with white/cream shell fragments; Luster: Dull or earthy; Structure: Bioclastic/Clastic; Cleavage: None; Specific Gravity: 2.3-2.7

Formation & Geological History

Formed in marine environments from the accumulation of organic debris such as shell fragments, corals, and algae. Over millions of years, chemical precipitation and pressure cement these fragments into rock. These often date from the Paleozoic to Cenozoic eras.

Uses & Applications

Used primarily in the construction industry as building stone, road base, and as a raw material for cement and lime production. Highly decorative specimens are used for architectural cladding or landscaping.

Geological Facts

Limestone often contains perfectly preserved fossils of ancient sea life. It is the primary rock found in karst landscapes, which are prone to sinkholes and caves. Coquina limestone was famously used to build the Castillo de San Marcos in Florida because its porous nature absorbed cannonballs.

Field Identification & Locations

Identify in the field by looking for visible shell fragments or fossils. It will react and fizz (effervesce) when exposed to a drop of dilute hydrochloric acid or strong vinegar due to the calcium carbonate content. Commonly found in coastal areas or regions that were once seabed.

Identified on: 5/15/2026

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