sedimentary
Chert (Iron-stained River Cobble)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, and orange-brown (from iron oxidation); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, and orange-brown (from iron oxidation)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, and orange-brown (from iron oxidation); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. This specific specimen shows 'smooth-weathering' and rounding indicative of transport in a high-energy river or stream environment. Geological age can range from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracturing. Currently used in road construction, as decorative landscaping stone, and occasionally for lapidary work (tumbling/polishing).
Geological facts
Chert is so durable that it often remains in riverbeds long after the surrounding softer rocks have eroded away. When struck against steel, it can produce sparks, leading to its historical use in flintlock firearms.
Field identification & locations
Identify in the field by its smooth 'river-worn' texture, hardness (it will scratch glass), and the way it breaks into sharp, curved flakes. Commonly found in riverbeds, gravel pits, and limestone outcrops globally.
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