sedimentary
Fossiliferous Chert (Gryphaea Fossil)
Silicon Dioxide (SiO2) with Calcitic Mollusk Remnants
Hardness: 6.5-7 Mohs; Color: Brown, tan, and white; Luster: Waxy to dull; Structure: Cryptocrystalline with visible biogenic inclusions; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Brown, tan, and white
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, and white; Luster: Waxy to dull; Structure: Cryptocrystalline with visible biogenic inclusions; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the fossilization of marine organisms where original calcium carbonate shells (likely of the extinct oyster genus Gryphaea) were partially or fully replaced by silica-rich groundwater. Most common in Mesozoic-era strata (Jurassic to Cretaceous periods).
Uses & applications
Used primarily for decorative landscaping, gravel, or as a collector's specimen. Historically used for making tools due to its sharp fracture edges.
Geological facts
Commonly known as 'Devil's Toenails' due to the distinctive curved, hoof-like shape of the Gryphaea shells trapped within. They were once believed in folklore to protect against arthritis.
Field identification & locations
Identify by looking for the 'Devil's Toenail' curve or layered shell structures embedded in a hard, glassy/waxy matrix. Common in the UK (Jurassic Coast) and parts of the United States like Texas. It will scratch glass and will not react to acid unless there is remaining calcite.
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