sedimentary

Chert Concretion

Silicon dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Tan, brown, or gray; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8

Hardness: 6
Color: Tan, brown, or gray
Luster: Waxy to dull

Identified June 5, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Tan, brown, or gray; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8

Formation & geological history

Formed through the precipitation of silica within porous sedimentary layers, typically limestone or chalk. These often grow around a nucleus like a fossil or a cavity during diagenesis millions of years ago.

Uses & applications

Historically used for making stone tools; today used in construction decorative stones and for geological education.

Geological facts

Chert is so hard it can scratch glass. It was one of the primary materials used by prehistoric humans to create arrowheads and scrapers due to its sharp edges when broken.

Field identification & locations

Identify in the field by its unusual, often bulbous or lumpy shape and high hardness compared to surrounding limestone. Frequently found in riverbeds or eroded limestone cliffs.