sedimentary
Chert (White-Tan variety)
Silicon dioxide (SiO2) / Microcrystalline Quartz
Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or off-white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine environments where silica replaces original carbonate minerals. Common in Phanerozoic rock layers.
Uses & applications
Historically used for stone tools and arrowheads because of its sharp edges. Currently used as road aggregate, in jewelry as semi-precious gemstones (when patterned), and as a silica source in glass manufacturing.
Geological facts
Chert is so hard it can strike sparks against steel, making it vital for early 'flintlock' firearms. Due to its microcrystalline structure, it breaks into razor-sharp edges which led to its extensive use in the Stone Age.
Field identification & locations
Identify by its waxy texture and shell-like (conchoidal) fracture pattern. It cannot be scratched by a steel knife. Common in limestone or chalk beds globally, specifically throughout the Midwestern and Western United States.
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