Chert (Jasper/Mudstone)

Physical Properties

Hardness: 6.5–7 on Mohs scale. Color: Brownish-tan to dark grey with waxy/dull luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6–2.7.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine or freshwater environments. This specimen shows signs of river or water tumbling, rounding its edges over thousands of years.

Uses & Applications

Commonly used as decorative stones in landscaping, historical use for stone tools (arrowheads) due to sharp fracture edges, and sometimes polished for jewelry or pocket stones.

Geological Facts

Chert is so durable that it often survives long after the surrounding rock has eroded away. Dark varieties like this are sometimes referred to as 'flint' if found in chalk or limestone deposits.

Field Identification & Locations

Identify in the field by its smooth, waxy texture and inability to be scratched by a steel knife. It often displays concave 'chip' marks known as conchoidal fractures. Common in riverbeds and coastal regions.

Identified on: 4/13/2026

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