sedimentary
Chert (Iron-stained)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale. Color: Tan, yellow-orange, or rusty brown due to iron oxide staining. Luster: Dull to waxy. Crystal structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
- Hardness
- 7 on the Mohs scale
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale. Color: Tan, yellow-orange, or rusty brown due to iron oxide staining. Luster: Dull to waxy. Crystal structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in marine or groundwater environments. It typically forms as nodules or bedded layers within limestone or chalk. Age varies from Precambrian to Cenozoic.
Uses & applications
Historically used for prehistoric stone tools (knapping) due to its sharp edges. Modernly used in construction aggregate, road surfacing, surfacing of abrasive tools, and occasionally as an ornamental stone in jewelry (as Jasper).
Geological facts
Chert is so hard it can strike sparks from steel, a property that led to its use in 'flintlock' firearms. When it is red or highly colored, it is often called Jasper; when it is translucent and gray/black, it is called Flint.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage pattern. Often found in riverbeds or weathered out of carbonate rock outcrops.
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