sedimentary

Chert (with Quartz veins)

Chert (Microcrystalline Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Black, gray, or dark brown with white/orange quartz veins; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Black, gray, or dark brown with white/orange quartz veins
Luster: Waxy to dull

Identified July 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary basins. The veins indicate later tectonic activity where silica-saturated water filled fractures.

Uses & applications

Historically used for stone tools (arrowheads and scrapers) due to its sharp edges. Modern use includes landscaping, road base, and as a variety of chalcedony for craft jewelry.

Geological facts

Black chert is often called flint. It is extremely chemically stable and resistant to weathering, which is why it often remains as large nodules or cobbles in soil and creek beds long after surrounding limestone has eroded.

Field identification & locations

Identify in the field by its conchoidal (shell-like) fracture pattern and its ability to scratch glass. Found globally in limestone formations and along riverbeds. Look for the distinct waxy texture compared to surrounding stones.

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