sedimentary

Chert (Septarian Nodules / Weathered Silex)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, or grey with weathered brown crust; Luster: Dull to waxy; Structure: Microcrystalline with deep desiccation cracks or 'septaria' patterns; Cleavage: None (conchoidal fracture).

Hardness: 6
Color: Tan, beige, or grey with weathered brown crust
Luster: Dull to waxy

Identified April 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich fluids within sedimentary rock layers, often limestone. The blocky, cracked appearance suggests it is a weathered nodule where internal shrinkage cracks or external environmental stress created the grid-like surface.

Uses & applications

Historically used for stone tools (knapping) due to its sharpness; currently used as garden decoration, road aggregate, and for geological study.

Geological facts

Chert is so durable that it often survives long after the surrounding limestone host rock has weathered away. This specimen shows 'elephant skin' weathering common in arid environments or areas with chemical etching.

Field identification & locations

Identify in the field by its high hardness (will scratch steel) and the presence of conchoidal (shell-like) fractures on broken edges. Commonly found in limestone outcrops or riverbeds.

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