sedimentary

Fossiliferous Limestone (Coquina / Shell Limestone)

Biogenic Limestone (Calcium Carbonate, CaCO3)

Hardness: 3 on Mohs scale; Color: Tan, cream, or light grey; Luster: Dull or earthy matrix with pearly shell fragments; Crystal Structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral in calcite particles; Specific Gravity: 2.3-2.7.

Hardness: 3 on Mohs scale
Color: Tan, cream, or light grey
Luster: Dull or earthy matrix with pearly shell fragments

Identified April 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in warm, shallow marine environments through the accumulation and compaction of shells, coral, and other organic debris. These often date back to the Cretaceous or Cenozoic periods.

Uses & applications

Used primarily in construction as road aggregate, in the manufacture of cement, and as a soil conditioner (lime). Decorative pieces are used in landscaping or fossil collecting.

Geological facts

This rock is essentially an ancient seabed. If you drop dilute hydrochloric acid on it, it will fizz (effervesce) vigorously because of the high calcium carbonate content. Many European cathedrals are built from similar bioclastic limestone.

Field identification & locations

Identify in the field by looking for visible shell impressions or fragments cemented in a fine-grained matrix. It is commonly found in regions that were once underwater, such as the US Gulf Coast or the Midwest.

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