sedimentary

Chert (Nodular)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, brown, and reddish; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, tan, brown, and reddish
Luster: Waxy to dull

Identified June 30, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, tan, brown, and reddish; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed primarily in marine environments where microcrystalline silicon dioxide precipitates from water or replaces other minerals within sedimentary beds like limestone. It can occur as nodules or layered beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in construction aggregate, and as a low-cost gemstone for lapidary work.

Geological facts

Chert is so hard that it can strike sparks from steel, which led to its use in early flintlock firearms. It is incredibly durable and resistant to chemical weathering.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass and steel), and smooth, shell-like (conchoidal) fractures. Found worldwide in sedimentary basins.