sedimentary
Chert (Jasper type)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Brown, reddish-brown, often mottled; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Brown, reddish-brown, often mottled
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Brown, reddish-brown, often mottled; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation from fluids within sedimentary host rocks. Common in ancient marine environments or as nodules in limestone/chalk.
Uses & applications
Historically used for stone tools and arrowheads; currently used as decorative gravel, in lapidary work (cabochons), and as sandpaper abrasive.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made it essential for early flintlock firearms. This specific specimen appears to be water-worn or stream-tumbled.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial tills, and outcrops of sedimentary rock layers.
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