sedimentary
Chert (specifically Jasper or Brown Chert node)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and dark streaks due to iron oxide impurities; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Brown, tan, and dark streaks due to iron oxide impurities
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and dark streaks due to iron oxide impurities; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often as nodules or replacement masses in limestone or chalk beds. This specimen shows signs of water-worn smoothing, suggesting secondary transport in a river or stream bed.
Uses & applications
Historically used for stone tools (flintknapping) due to its sharp edges; modern uses include aggregate in construction, landscaping, and as semi-precious gemstones (Jasper) for jewelry and tumbling.
Geological facts
Chert is so durable that it often survives long after the surrounding 'mother' rock has eroded away. Because it breaks with a conchoidal (shell-like) fracture, it was the primary material for prehistoric arrowheads and scrapers.
Field identification & locations
Identify in the field by its waxy luster, extreme hardness (it will scratch glass and steel), and smooth, curved fracture surfaces. It is commonly found in gravel pits, riverbeds, and as nodules protruding from weathered limestone outcrops.
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