sedimentary
Coquina
Biogenic Limestone (Calcium Carbonate - CaCO3)
Hardness: 3 (Mohs scale); Color: Cream, tan, or grayish-white; Luster: Relates to organic matter, often dull or pearly; Crystal structure: Orthorhombic/Trigonal (individual shells); Cleavage: Distinct in calcite crystals; Specific Gravity: 2.3–2.4 (quite porous).
- Hardness
- 3 (Mohs scale)
- Color
- Cream, tan, or grayish-white
- Luster
- Relates to organic matter, often dull or pearly
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Physical properties
Hardness: 3 (Mohs scale); Color: Cream, tan, or grayish-white; Luster: Relates to organic matter, often dull or pearly; Crystal structure: Orthorhombic/Trigonal (individual shells); Cleavage: Distinct in calcite crystals; Specific Gravity: 2.3–2.4 (quite porous).
Formation & geological history
Formed in high-energy marine environments (beaches or shallow water) where wave action accumulates shell fragments. Over time, these fragments are chemically cemented together by calcite. Most deposits are relatively young (Pleistocene to Holocene ages).
Uses & applications
Historically used as a building material for forts and homes due to its ability to absorb cannon fire (it compresses rather than shattering). Today used in landscaping, road base, and as a novelty stone for collectors.
Geological facts
The Castillo de San Marcos in St. Augustine, Florida, is the most famous structure built from coquina; it successfully survived numerous sieges because the stone's porous nature allowed it to absorb metal cannonball impacts.
Field identification & locations
Identify by looking for a rock composed entirely of visible shell fragments or coral pieces glued together. Commonly found along the coastlines of Florida and the Caribbean. Collectors look for pieces with well-preserved, intact shell structures.
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