sedimentary
Chert (Iron-stained)
Chert (Cryptocrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Rusty brown, black, and tan (iron-stained); Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: ~2.6.
- Hardness
- 7 (Mohs scale)
- Color
- Rusty brown, black, and tan (iron-stained)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Rusty brown, black, and tan (iron-stained); Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: ~2.6.
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation in marine environments. The reddish coloring is due to iron oxide staining during or after formation.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges. Modernly used in landscaping, as road base, or as tumbled decorative stones.
Geological facts
Chert is so hard it can strike sparks against steel, a property that led to its use in flintlock firearms. This specific specimen shows typical stream-tumbling wear, giving it a rounded, smooth exterior.
Field identification & locations
Identify by its extreme hardness (will scratch glass), waxy texture, and conchoidal (shell-like) fracture patterns. Found globally in sedimentary basins and riverbeds.
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