Coquina

Physical Properties

Hardness: 2-3 (Mohs scale); Color: Tan, cream, or brown; Luster: Dull to pearly; Structure: Fragmental/Clastic (composed of shell fragments); Cleavage: None (friable/crumbly); Specific Gravity: 1.5 - 2.5

Formation & Geological History

Formed in high-energy marine environments like beaches or reefs where wave action accumulates broken shell fragments. Over time, these fragments are naturally cemented together by calcium carbonate. Most specimens found today are from the Pleistocene or Holocene epochs.

Uses & Applications

Historically used as a building stone (notable in old forts) because its porous nature absorbs cannonball impacts. Currently used in landscaping, as road base material, and as a calcium supplement for livestock.

Geological Facts

The Castillo de San Marcos in St. Augustine, Florida, is the most famous structure built of coquina. Because the rock is porous, cannonballs would bury themselves in the walls rather than shattering them.

Field Identification & Locations

Identify by the presence of visible, often identifiable shell fragments or coral bits that are loosely or moderately cemented together. It feels rough and often looks like a piece of granola made of shells. Commonly found along coastal areas, especially in Florida and the Carolinas.

Identified on: 4/15/2026

Mode: Standard