sedimentary

Chert

Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Grey, black, or greenish with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5-2.8.

Hardness: 6
Color: Grey, black, or greenish with waxy luster

Identified June 10, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Grey, black, or greenish with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5-2.8.

Formation & geological history

Formed through the accumulation of silica-rich microfossils or chemical precipitation within marine or groundwater environments. This specimen appears to have been weathered and rounded in a glacial or fluvial setting.

Uses & applications

Historically used for stone tools (arrowheads). Today used as road aggregate, architectural stone, and occasionally in lapidary work.

Geological facts

Chert is so hard it can create sparks when struck against steel. It is nearly chemically inert and extremely resistant to weathering.

Field identification & locations

Identify by its waxy luster, hardness (cannot be scratched by steel), and smooth shell-like (conchoidal) fractures. Found globally in limestone beds or as rounded river/beach cobbles.