sedimentary
Fossilized Oyster Shell
Gryphaea or Crassostrea fossil (Calcium Carbonate, CaCO3)
Hardness: 3 (Mohs scale), Color: Brown, gray, tan, Luster: Dull to pearly inside, Crystal Structure: Trigonal (as calcite or aragonite), Cleavage: Perfect, Specific Gravity: 2.7
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 3 (Mohs scale), Color: Brown, gray, tan, Luster: Dull to pearly inside, Crystal Structure: Trigonal (as calcite or aragonite), Cleavage: Perfect, Specific Gravity: 2.7
Formation & geological history
Formed through the process of permineralization or replacement where an ancient oyster's calcium carbonate shell is buried in sediment and chemically altered over millions of years, often in marine environments from the Mesozoic to Cenozoic eras.
Uses & applications
Used primarily as educational specimens, biological research, collector items, and occasionally crushed for high-calcium fertilizer or industrial lime if found in massive quantities.
Geological facts
Commonly known as 'Devil's Toenails' (specifically for the genus Gryphaea), these fossils indicate that the region where they were found was once under a shallow sea. Some fossils contain druzy quartz or calcite crystals that grew inside the cavity after death.
Field identification & locations
Identify by visible growth lines (lamellae), calcified texture, and the distinctive 'hinge' area of the bivalve. Found in sedimentary deposits like limestone or shale cliffs and riverbeds.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary