sedimentary
Fossiliferous Limestone (Coquina / Shell Bed)
Biogenic Limestone (Calcium Carbonate, CaCO3)
Hardness: 3-4 (Mohs scale); Color: Buff, tan, or yellowish-brown; Luster: Dull to earthy; Crystal structure: Non-crystalline matrix with visible fossil shells; Cleavage: None; Specific gravity: 2.3–2.7.
- Hardness
- 3-4 (Mohs scale)
- Color
- Buff, tan, or yellowish-brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Buff, tan, or yellowish-brown; Luster: Dull to earthy; Crystal structure: Non-crystalline matrix with visible fossil shells; Cleavage: None; Specific gravity: 2.3–2.7.
Formation & geological history
Formed in shallow marine environments where skeletal debris (shells) accumulated and were subsequently cemented together by calcite. Often dates to the Paleozoic or Mesozoic eras.
Uses & applications
Primary source of lime and cement; historically used as a building stone (e.g., in Florida for fort construction) due to its shock-absorbing quality. Smaller pieces are popular as garden accents or educational fossil specimens.
Geological facts
Famous structures like the Castillo de San Marcos in St. Augustine, Florida, were built using a similar shell limestone called Coquina, which proved resistant to cannon fire because it compressed rather than shattered.
Field identification & locations
Identify by visible brachiopod, bivalve, or gastropod shell impressions/fragments. It will react and fizz strongly when exposed to dilute hydrochloric acid (acid test). Commonly found in outcrop regions of ancient seabeds, such as the Midwestern United States or coastal plains.
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