Coquina

Physical Properties

Hardness: 2-3 (Mohs scale), Color: Tan, beige, grey, or whitish, Luster: Dull to earthy, Crystal Structure: Amorphous (shells), Cleavage: None, Specific Gravity: 1.5 - 2.5

Formation & Geological History

Formed in high-energy marine environments like beaches and reefs where shells are broken by wave action and naturally cemented together by calcium carbonate. Most specimens date from the Pleistocene or Holocene epochs.

Uses & Applications

Used historically in Florida and the Caribbean as a building stone (e.g., Castillo de San Marcos) because it absorbs cannon fire without shattering. Also used for driveway gravel and as an additive in livestock feed.

Geological Facts

Because it is composed of poorly cemented shell fragments, it is very porous and soft when first quarried, but it hardens over time as it dries and minerals precipitate.

Field Identification & Locations

Identified in the field by its 'oatmeal' or 'shredded wheat' appearance, clearly visible shell fragments, and strong reaction (fizzing) when exposed to dilute hydrochloric acid. Found primarily along coastlines.

Identified on: 4/20/2026

Mode: Standard