sedimentary
Chert Concretion (with typical hematite/limonite staining)
Sedimentary Concretion (SiO2 with FeO(OH)⋅nH2O)
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or dark brown/black; Luster: Dull to earthy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High durability.
- Hardness
- 6
- Color
- Gray, tan, or dark brown/black
- Luster
- Dull to earthy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or dark brown/black; Luster: Dull to earthy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High durability.
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between particles in sedimentary rock, often around a nucleus like a shell or leaf. This specific specimen exhibits 'pot-hole' or 'paint pot' weathering where a softer inner core has eroded out, leaving a bowl-like shell.
Uses & applications
Primarily used as a geological curiosity or 'curio' for collectors. Historically, chert was used for tool making, but concretions are often too irregular. Occasionally used in rock gardens or as natural decor.
Geological facts
These are often mistaken for man-made artifacts like ancient bowls or 'paint pots' used by indigenous peoples, but they are naturally occurring geological formations. They can range from the size of a marble to several meters wide.
Field identification & locations
Identify by its bowl-like or spherical shape and extreme hardness compared to surrounding sandstone or shale. Often found in areas with heavy sedimentary erosion, particularly in riverbeds or desert pavement.
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