sedimentary
Chert (specifically Tan/Yellow Chert)
Chert (Microcrystalline or Cryptocrystalline Silica, SiO2)
Hardness: 7 on the Mohs scale; Color: Tan, beige, or yellowish-brown; Luster: Waxy to dull; Structure: Cryptocrystalline (crystals too small to see with a microscope); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Tan, beige, or yellowish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on the Mohs scale; Color: Tan, beige, or yellowish-brown; Luster: Waxy to dull; Structure: Cryptocrystalline (crystals too small to see with a microscope); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical precipitation of silica in marine environments. It often replaces original limestone or chalk through a process called diagenesis. Found in geological formations ranging from the Precambrian to the Cenozoic era.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal edges. Modern uses include construction aggregate, road surfacing, and as a semi-precious gemstone (when colored/patterned as jasper).
Geological facts
Chert is so hard it can scratch steel and produce sparks when struck against iron, which led to its historical use in flintlock firearms. 'Flint' is actually a high-quality variety of dark chert.
Field identification & locations
Identify in the field by its characteristic 'shell-like' shell-like (conchoidal) fractures, its extreme hardness, and its smooth, waxy surface. It won't react with acid (unlike limestone) and is commonly found as nodules or layers within sedimentary rock beds throughout the Midwestern USA and Europe.
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