sedimentary

Chert (containing Fossil Mold)

Silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs scale; Color: Cream, tan, grey; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture)

Hardness: 6
Color: Cream, tan, grey
Luster: Dull to waxy

Identified May 31, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs scale; Color: Cream, tan, grey; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture)

Formation & geological history

Formed from the accumulation of silica-rich organic remains or chemical precipitation in marine environments, often replacing original limestone. These can date from the Precambrian to recent eras.

Uses & applications

Historically used for stone tools/arrowheads. Modern uses include abrasives, road construction, and as aquarium or landscape decor.

Geological facts

Chert is so hard it can spark when struck against steel. The indentation in this specimen appears to be a fossil mold where a shell or organic matter was once embedded before dissolving.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel pocketknife) and its smooth, shell-like fracture surfaces. Frequently found in limestone outcrops.