Chert (specifically Yellow or Brown Chert / Jasper)

Physical Properties

Hardness: 7 (Mohs scale). Color: Yellow-brown to tan (due to iron oxide inclusions). Luster: Waxy to dull. Crystal Structure: Cryptocrystalline (microscopic quartz). Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary layers. These specimens can range from Precambrian to Cenozoic in age.

Uses & Applications

Historically used for stone tools and arrowheads. Currently used as road aggregate, decorative landscaping, and occasionally lapidary work (cabbing) if it has high silica content and takes a polish.

Geological Facts

Chert is so hard it can strike sparks from steel, which is why variants like flint were essential for starting fires and for early firearms. It is incredibly durable and resistant to chemical weathering.

Field Identification & Locations

Identify in the field by its conchoidal (shell-like) fracture patterns and ability to scratch glass easily. Look for it in riverbeds or as nodules/layers within limestone. Collectors should look for unique banding or translucency.

Identified on: 4/13/2026

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