sedimentary
Chert (Nodule / Geode fragment)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: White to tan/cream; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- White to tan/cream
- Luster
- Waxy to dull/earthy
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Physical properties
Hardness: 7 on Mohs scale; Color: White to tan/cream; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica within limestone or chalk beds, often replacing organic material or filling cavities. This specific specimen appears to be a weathered nodule fragment or a partial geode shell, likely originating from the Mesozoic or Cenozoic era.
Uses & applications
Historically used for stone tools and fire-starting (flint); currently used as an abrasive, in road construction (crushed stone), and as a landscape rock. Some attractive specimens are used for jewelry (cabochons).
Geological facts
Chert is so chemically stable that it often remains as 'river gravel' long after the surrounding limestone has eroded. It was the primary material for spearheads and arrowheads due to its predictable conchoidal fracturing creating razor-sharp edges.
Field identification & locations
Identify by its 'cup' shape suggesting a hollow center, its ability to scratch glass (hardness), and its smooth, waxy surface. Often found in riverbeds or eroding out of limestone cliffs. Best found in regions with significant limestone deposits like the American Midwest or the English Cotswolds.
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