sedimentary

Chert (Agatized)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs). Color: Tan, brown, and white. Luster: Waxy to vitreous. Crystal Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.5-2.8.

Hardness: 6
Color: Tan, brown, and white
Luster: Waxy to vitreous

Identified July 14, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs). Color: Tan, brown, and white. Luster: Waxy to vitreous. Crystal Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.5-2.8.

Formation & geological history

Formed through the precipitation of silica from groundwater within sedimentary rock cavities or by the replacement of calcium carbonate. This specimen appears water-worn, likely tumbled in a river or glacial deposit.

Uses & applications

Primarily used as a lapidary material for tumbling and cabochons. Historically used for making stone tools (flintknapping) due to its sharp fracture edges.

Geological facts

Chert is one of the most common sedimentary rocks and has been used by humans for millions of years. This specific piece shows fine banding or 'fortification' lines common in chalcedony/agate varieties.

Field identification & locations

Identify by its waxy texture, inability to be scratched by a steel knife, and smooth, rounded surface indicating transport by water. Common in riverbeds and gravel pits across North America.