sedimentary
Coquina / Shelly Limestone
Bioclastic Limestone (composed of CaCO3)
Hardness: 3 (Mohs scale); Color: tan, white, and rusty orange due to iron oxidation; Luster: dull/earthy; Crystal structure: varied (amorphous shell fragments); Cleavage: none; Specific gravity: 2.3-2.7
- Hardness
- 3 (Mohs scale)
- Color
- tan, white, and rusty orange due to iron oxidation
- Luster
- dull/earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: tan, white, and rusty orange due to iron oxidation; Luster: dull/earthy; Crystal structure: varied (amorphous shell fragments); Cleavage: none; Specific gravity: 2.3-2.7
Formation & geological history
Formed in high-energy marine environments like beaches or near-shore reefs where shell fragments accumulate and are naturally cemented together by calcite. Typically found in deposits from the Pleistocene or Holocene epochs.
Uses & applications
Historically used as a building material for forts (like Castillo de San Marcos) because it absorbs cannonball impacts; used today in road base, landscape stone, and as a source of lime.
Geological facts
Coquina is unique because it is soft enough to be cut with a saw when first quarried but hardens significantly after exposure to air. The Castillo de San Marcos in Florida survived many sieges because the stone's porous nature allowed it to swallow cannonballs rather than shattering.
Field identification & locations
Identify in the field by its visible 'macroscopic' shell fragments (mollusks, trilobites, or coral) and its reactive bubbling when exposed to dilute hydrochloric acid. Commonly found along the Atlantic coast of Florida and North Carolina.
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