sedimentary
Chert (River Pebble)
Microcrystalline silicon dioxide (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Grayish-green with brownish iron staining; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 6
- Color
- Grayish-green with brownish iron staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Grayish-green with brownish iron staining; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich microfossils or chemical precipitation from groundwater. This specimen appears water-worn, indicating it was transported by a river or stream, likely dating from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools and sparks (flint). Today used as aggregate in construction, decorative landscaping, and occasionally for lapidary tumbling.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has weathered away. It is chemically identical to quartz but has a much finer grain structure.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, curved conchoidal fractures. Commonly found in riverbeds and within limestone or chalk formations.
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