sedimentary
Chert (River Cobble)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, tan, and reddish-grey; Luster: Dull to waxy; Structure: Non-crystalline/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown, tan, and reddish-grey
- Luster
- Dull to waxy
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 7 (Mohs scale); Color: Brown, tan, and reddish-grey; Luster: Dull to waxy; Structure: Non-crystalline/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation. This specific specimen has been chemically weathered and physically rounded into a 'cobble' by water action in a river or creek bed.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp edges when broken. Today used in landscape construction, road base, and as decorative river rock.
Geological facts
Chert is so hard it can scratch steel; when struck against steel, it can create sparks, which led to its historical use in flintlock firearms and fire-starting.
Field identification & locations
Identify by its extreme hardness, smooth water-worn surface, and waxy luster on fresh breaks. Found globally in stream beds, gravel pits, and sedimentary outcrops.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary