sedimentary

Chert (Agatized)

Silicon dioxide (SiO2)

Hardness: 7 Mohs; Color: Tan, cream, and brown with gray translucency; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 Mohs
Color: Tan, cream, and brown with gray translucency
Luster: Waxy to vitreous

Identified May 3, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Tan, cream, and brown with gray translucency; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich microfossils or chemical precipitation within sedimentary rock layers, often replacing limestone or dolostone during diagenesis.

Uses & applications

Used for making jewelry (cabochons), historically used for stone tools due to sharp edges, and occasionally used as a decorative landscaping stone.

Geological facts

Chert is incredibly durable and chemically stable, which is why ancient civilizations used it for arrowheads and spear points. This specimen shows slight water-worn smoothing and partial agatization.

Field identification & locations

Identify in the field by its waxy luster, conchoidal (shell-like) fracture, and inability to be scratched by a steel knife. Commonly found in riverbeds, limestone outcrops, and coastal areas.