sedimentary
Chert (Smooth River Pebble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Gray to olive-green with banding; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- Gray to olive-green with banding
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray to olive-green with banding; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from seawater or the accumulation of siliceous skeletal remains (like diatoms or radiolarians). This specimen has been rounded and smoothed by water erosion in a fluvial (river) or coastal environment.
Uses & applications
Used historically for stone tools and starting fires; currently used as construction aggregate, road surfacing, and occasionally for lapidary work like rock tumbling.
Geological facts
Chert is so hard it can scratch steel. Historically, varieties like flint were essential for survival as they produce sparks when struck against iron or pyrite. The banding seen here suggests rhythmic deposition of different mineral impurities.
Field identification & locations
Identify in the field by its extreme hardness, smooth waxy texture, and the way it breaks into sharp, curved 'glass-like' edges. Found globally in sedimentary basins and as river 'cobbles'. Excellent for beginner rock tumbling.
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