sedimentary
Chert (Iron-stained)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brownish-yellow to tan due to iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific Gravity: 2.6 – 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brownish-yellow to tan due to iron oxide staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brownish-yellow to tan due to iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific Gravity: 2.6 – 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine or groundwater environments; commonly found in limestone or chalk beds, often from the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads and scrapers) due to its sharp edges; modern uses include construction aggregate, gemstones (when colorful), and a source of silicon.
Geological facts
Chert is so hard it can strike sparks against steel to start fires. It is nearly chemically inert and extremely resistant to weathering, which is why it is often found as loose stones in riverbeds long after the surrounding limestone has eroded.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel nail), conchoidal (shell-like) fracture patterns, and waxy texture. Commonly found in gravel pits, riverbeds, and as nodules in sedimentary rock formations.
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