sedimentary
Chert (Iron-stained)
Chert (Cryptocrystalline Silica, SiO2)
Hardness: 7 (Mohs); Color: Brown, tan, and white; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Fracture: Conchoidal; Specific gravity: 2.65
- Hardness
- 7 (Mohs)
- Color
- Brown, tan, and white
- Luster
- Waxy to dull/earthy
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Physical properties
Hardness: 7 (Mohs); Color: Brown, tan, and white; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Fracture: Conchoidal; Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. This specimen shows iron oxidation (rusting) giving it a brown hue.
Uses & applications
Historical use for stone tools and flintknapping. Modern use in construction aggregate, road base, and occasionally as an ornamental garden stone.
Geological facts
Chert is so hard it can scratch glass and steel. Historically, varieties like flint were essential for starting fires and weaponry due to their sharp, predictable fracture patterns.
Field identification & locations
Identify by its extreme hardness and smooth, shell-like fracture surfaces if broken. It is often found as nodules or layers within sedimentary rock outcrops or as river-worn cobbles.
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