sedimentary

Chert (River Cobble)

Chert (Cryptocrystalline Quartz, SiO2)

AI-generated identification · may be incorrect

Mohs Hardness: 7, Color: Tan, brown, and yellowish-orange due to iron oxides, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.6-2.7

Identified June 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica from water, often in marine environments or by replacing organic material. This specimen is a stream-rounded cobble, likely eroded from a limestone or dolomite source rock and transported by water.

Uses & applications

Historically used for stone tools (arrowheads, scrapers). Currently used as construction aggregate, road base, and occasionally for decorative landscaping.

Geological facts

Chert is essentially a dense, opaque variety of quartz. When it is red, it is called Jasper; when it is translucent and banded, it is called Agate. It is extremely chemically stable and resistant to weathering.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture when broken. Look for it in riverbeds, gravel pits, and sedimentary rock outcroppings.