sedimentary
Chert (specifically a stream-worn Chert Nodule)
Sedimentary Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, or grey with possible iron staining; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, or grey with possible iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, or grey with possible iron staining; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); 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 within limestone or chalk beds. Often secondary replacement of carbonate minerals. These specific specimens are stream-tumbled, likely from the Holocene era but originating from much older Paleozoic or Mesozoic bedrock.
Uses & applications
Historically used for stone tools (flintknapping); modern uses include road surfacing, decorative garden stone, and lapidary work (tumbling/polishing).
Geological facts
Chert is so durable that it often survives as 'lag' long after the surrounding limestone has eroded away. It was one of the most important resources for prehistoric humans to create sharp-edged tools and arrowheads.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy texture when wet, and smooth, rounded water-worn surface. Often found in riverbeds or gravel bars. Collectors look for unique banding or fossils trapped inside.
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