sedimentary
Chert Nodule (Fossiliferous Chert)
Microcrystalline Quartz (SiO2)
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 More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary