sedimentary
Chert (often called Jasper or River Siltstone)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Brown, tan, and dark olive/black; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
- Hardness
- 6
- Color
- Brown, tan, and dark olive/black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Brown, tan, and dark olive/black; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils or chemical precipitation from fluids within sedimentary rock layers. This specimen has been water-worn into a smooth river cobble, likely over millions of years.
Uses & applications
Used historically for stone tools due to sharp fracture edges; currently used for decorative landscaping, lapidary items (jewelry cabochons), and as aquarium stones.
Geological facts
The distinct color banding or dark patches are often caused by iron oxides or organic impurities. Chert is so hard it can create sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, waxy texture. Commonly found in riverbeds, glacial tills, or embedded in limestone formations globally.
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