sedimentary
Chert (Iron-stained)
Chert (primarily Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Gray to tan with yellow-brown iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray to tan with yellow-brown iron oxide staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to tan with yellow-brown iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from silica-rich fluids. Often found as nodules in limestone or chalk beds spanning various geological periods from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modern uses include road construction aggregate, landscaping, and occasionally as an abrasive.
Geological facts
Chert is so hard it can create sparks when struck against steel, a property that led to its use in flintlock firearms. The yellowish-orange tint on this specimen is likely due to the oxidation of iron-bearing minerals (limonite or goethite).
Field identification & locations
Identify in the field by its extreme hardness (will scratch glass), waxy luster on fresh surfaces, and characteristic smooth, curved (conchoidal) breaks. Very common in sedimentary basins worldwide.
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