Chert (often called Jasper or River Siltstone)

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.

Identified on: 4/28/2026

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