sedimentary

Banded Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Tan, yellow, and brown bands; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 7 Mohs
Color: Tan, yellow, and brown bands
Luster: Waxy to dull

Identified July 3, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Tan, yellow, and brown bands; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & geological history

Formed primarily as a chemical precipitate from silica-rich fluids in marine sedimentary environments where micro-organisms like diatoms or radiolarians accumulate. The bands represent different concentrations of iron oxides and clay impurities during deposition.

Uses & applications

Historically used for making sharp tools like arrowheads; currently used as aquarium decor, garden stones, and sometimes polished for decorative lapidary work.

Geological facts

Chert is so hard it can strike sparks from steel. Banded varieties like this are often found in riverbeds where water has smoothed the edges but preserved the internal layering.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel pocket knife) and smooth, waxy surface. Look for it in gravel pits, riverbeds, or as nodules within limestone outcrops.