sedimentary
Chert Nodules (with Gryphaea or Shell inclusions)
Microcrystalline Quartz (SiO2) with Calcitic inclusions
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Gray, tan, off-white
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed primarily in marine environments through the replacement of limestone or carbonate sediment by silica-rich fluids. Often contains fossilized remnants ranging from the Mesozoic or Cenozoic eras.
Uses & applications
Used historically for stone tools and strike-a-lights. Today primarily of interest to hobbyist collectors and used as garden decoration or road aggregate.
Geological facts
This specimen appears to be a weathered beach pebble of chert or flint containing fossilized shell fragments, possibly 'Devil\u0027s Toenails' (Gryphaea) which were common in ancient shallow seas.
Field identification & locations
Identify by its hardness (it will scratch glass) and smooth, rounded surface indicating water wear. Commonly found on beaches and in riverbeds where limestone bedrock is present.
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