sedimentary

Chert (Brecciated or Veined)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, and tan/white; Luster: Waxy to dull; Crystal structure: Microcrystalline (cryptocrystalline); Cleavage: None (has conchoidal fracture); Specific gravity: 2.6 – 2.7.

Hardness: 7 on Mohs scale
Color: Gray, black, and tan/white
Luster: Waxy to dull

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater. This specimen shows brecciation, where original rock fractured and was later 'cemented' or veined by secondary silica infilling during diagenesis or tectonic activity.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal edges; currently used for road aggregate, construction fill, and occasionally as an ornamental lapidary material when patterned.

Geological facts

Chert is so durable that it often survives long after the surrounding rock has weathered away. It was one of the first minerals ever 'mined' by prehistoric humans to create complex tools and weaponry.

Field identification & locations

Identify in the field by its hardness (it can scratch glass and steel), its waxy texture, and its shell-like (conchoidal) fracture pattern. It is commonly found in limestone layers, as nodules in chalk, or as stream-worn river pebbles.

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