sedimentary

Chert / Chalcedony

Silicon Dioxide (SiO2)

Hardness: 6.5 - 7 on the Mohs scale. Color: Grayish-brown, tan, to semi-translucent smoke. Luster: Waxy to sub-vitreous. Fracture: Conchoidal with sharp edges. Cleavage: None.

Hardness: 6
Color: Grayish-brown, tan, to semi-translucent smoke
Luster: Waxy to sub-vitreous

Identified July 27, 2026More sedimentary →

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Physical properties

Hardness: 6.5 - 7 on the Mohs scale. Color: Grayish-brown, tan, to semi-translucent smoke. Luster: Waxy to sub-vitreous. Fracture: Conchoidal with sharp edges. Cleavage: None.

Formation & geological history

Formed by the precipitation of silica from aqueous solutions, often in marine or groundwater sedimentary environments, replacing organic materials or forming nodules within limestone and chalk beds.

Uses & applications

Historically used for making stone tools such as arrowheads, scrapers, and fire-starters due to its sharp conchoidal fracture. Today used as road aggregate, decorative stone, and in lapidary arts.

Geological facts

Chert and flint were among the earliest materials used by ancient humans to create tools and spark fires. Its cryptocrystalline structure makes it extremely durable and sharp when fractured.

Field identification & locations

Can be identified in the field by its hardness (scratches glass), smooth waxy feel, conchoidal fracture patterns, and lack of visible mineral grains. Very common in riverbeds, gravel deposits, and sedimentary rock formations worldwide.