sedimentary
Fossil Shell (Bivalve)
Class Bivalvia (Likely a member of the Ostreidae or Gryphaeidae family), CaCO3
Hardness: 3-3.5 (Mohs scale); Color: Cream, tan, or yellowish-white; Luster: Pearly to dull; Crystal Structure: Trigonal (calcite/aragonite); Cleavage: Perfect; Specific Gravity: 2.7-2.9.
- Hardness
- 3-3
- Color
- Cream, tan, or yellowish-white
- Luster
- Pearly to dull
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Physical properties
Hardness: 3-3.5 (Mohs scale); Color: Cream, tan, or yellowish-white; Luster: Pearly to dull; Crystal Structure: Trigonal (calcite/aragonite); Cleavage: Perfect; Specific Gravity: 2.7-2.9.
Formation & geological history
Formed through the fossilization of marine mollusks. This occurs via permineralization or replacement where mud and minerals encase the shell in sedimentary rock layers. Common in strata from the Jurassic to the Neogene periods.
Uses & applications
Educational specimens for geology students, decorative items in aquarium/terrarium trade, jewelry as 'organic gems', and use in cement or lime production when found in large limestone deposits.
Geological facts
Bivalves have existed for over 500 million years. Famous specimens like 'Devil’s Toenails' (Gryphaea) were used in folk medicine, and fossils like these help geologists date specific sedimentary layers via biostratigraphy.
Field identification & locations
Identify by looking for concentric growth lines (ribs) and a hinge point (beak). Commonly found in eroded limestone outcroppings, riverbeds, or coastal cliff faces. Collectors should look for symmetry and the presence of both valves (halves) joined together.
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