sedimentary
Chert (specifically Yellow or River Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or mustard; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Yellow-brown, tan, or mustard
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or mustard; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed from the precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. Many specimens date from the Paleozoic or Mesozoic eras and are often found as water-worn cobbles in riverbeds.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp edges; modernly used as road aggregate, in landscaping, and as a low-cost material for lapidary tumbling.
Geological facts
Chert is so hard it can strike sparks against steel, making it a primitive 'fire-starter.' Yellow varieties often owe their color to traces of iron oxide (limonite) within the silica matrix.
Field identification & locations
Identify in the field by its smooth, waxy texture when wet and its distinctive 'shell-like' conchoidal fracture. It will scratch glass easily. Commonly found in riverbeds, gravel pits, and eroded limestone outcrops.
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