sedimentary
Chert (Brecciated River Cobble)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: Cream, tan, with greenish grey veins, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.65
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Cream, tan, with greenish grey veins, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or freshwater environments. This specific specimen shows brecciation (cracking and re-cementing) and has been rounded by fluvial (river) erosion.
Uses & applications
Primarily used for landscape gravel and road base in aggregate form. Historically used for stone tools. Occasional use in lapidary work if it takes a high polish.
Geological facts
Chert is so tough that it can spark when struck against steel. Brecciated chert tells a story of tectonic or chemical stress that shattered the rock before it was glued back together by more silica.
Field identification & locations
Identify by its hardness (it will scratch glass) and waxy texture. Commonly found in riverbeds and as nodules within limestone or chalk formations. Look for the irregular, angular internal patterns contrasting with a smooth outer surface.
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