sedimentary

Chert (Smooth River Pebble)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified March 29, 2026More sedimentary →

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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.