sedimentary
Chert Concretion
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, and yellowish-white; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High durability.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, and yellowish-white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, and yellowish-white; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High durability.
Formation & geological history
Formed by the chemical precipitation of silica within sedimentary rock layers, often limestone or chalk. These nodules grow around a nucleus (like a shell fragment) during diagenesis as mineral-rich fluids circulate through pore spaces.
Uses & applications
Primarily of interest to collectors of oddities and hag stones; historically used for stone tools (flintknapping) due to its sharp edges when broken.
Geological facts
This specific specimen has a bulbous, concretionary shape that often leads people to mistake them for fossils or man-made artifacts. Chert is one of the most chemically stable minerals at the Earth's surface.
Field identification & locations
Identified by its extreme hardness (it will scratch glass) and smooth, waxy texture compared to surrounding limestone. Often found in riverbeds or coastal areas where softer host rock has eroded away.
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