sedimentary
Banded Chert (River Stone/Cobble)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan and dark grey/black bands; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan and dark grey/black bands
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan and dark grey/black bands; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or lacustrine environments. This specimen has been rounded into a cobble by water erosion in a river or coastal setting over thousands of years.
Uses & applications
Used historically for stone tools and arrowheads; modernly used as decorative garden stone, in road aggregate, or as lapidary material for tumbling and polishing.
Geological facts
Chert is so hard it can scratch steel and was once commonly used to start fires when struck against iron; it is chemically identical to flint and jasper.
Field identification & locations
Identify by its 'waxy' texture when broken, extreme hardness, and smooth, rounded water-worn surface. Commonly found in riverbeds, glacial tills, and coastal beaches globally.
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