sedimentary
Chert (or Chalcedony) Nodule
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Tan, beige, or yellowish-brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan, beige, or yellowish-brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 Mohs; Color: Tan, beige, or yellowish-brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed in sedimentary environments, often replacing carbonate material or precipitating from silica-rich fluids within limestone or chalk beds. Commonly found in Paleozoic or Mesozoic age strata.
Uses & applications
Historically used for stone tools due to sharp edges; currently used for road base, decorative landscaping, and occasionally lapidary art if high quality.
Geological facts
This specimen appears to be a weathered chert nodule, likely tumbled by water. Chert is so durable that it often remains long after the surrounding limestone host rock has eroded away.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy texture when broken, and lack of reaction to acid. Often found in riverbeds or gravel pits.
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