sedimentary

Chert Nodule (Fossiliferous Chert)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs); Color: Tan, grey, and honey-brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 6
Color: Tan, grey, and honey-brown
Luster: Waxy to vitreous

Identified February 20, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: Tan, grey, and honey-brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the precipitation of silica from groundwater within limestone or chalk beds. Often replaces organic matter or fills voids; these specimens typically date from the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (flintknapping); modern uses include landscaping, road aggregate, and as a semi-precious gemstone when polished as 'Chalcedony' or 'Agate'.

Geological facts

Chert nodules often form around a biological nucleus, such as a fossil fragment or sponge. This specimen appears to have a central 'core' which may be a fossilized stalk or a mineral-filled geode-like cavity.

Field identification & locations

Identify by its 'rind' (weathered outer crust) and sharp, glass-like fracture surfaces when broken. Common in marine sedimentary basins such as those in the Midwest US and Southern UK.