sedimentary

Chert Concretion

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs scale; Color: Beige, tan, or off-white; Luster: Dull to waxy; Structure: Cryptocrystalline/Massive (no visible crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs scale
Color: Beige, tan, or off-white
Luster: Dull to waxy

Identified July 21, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Beige, tan, or off-white; Luster: Dull to waxy; Structure: Cryptocrystalline/Massive (no visible crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed in sedimentary rock layers (like limestone or chalk) through the chemical precipitation of silica-rich fluids. Silica replaces organic or mineral matter, creating a hard, dense nodule or concretion. Ages vary from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction as gravel, as a grinding medium in ball mills, and occasionally as decorative landscaping stone.

Geological facts

Chert nodules often form around a nucleus, such as a fossil or a piece of organic debris. In ancient times, it was the primary material for making arrowheads and spear points because of its sharp conchoidal fracture.

Field identification & locations

Identified in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, waxy feel when freshly broken. Found globally in limestone and dolostone formations. Use an acid test: it will not fizz like the limestone it is often found within.