sedimentary

Chert (Nodule)

Chert (Alpha-Quartz, SiO2)

Hardness: 7.0 on Mohs scale; Color: Dark brown, black, to tan; Luster: Waxy to sub-vitreous; Crystal Structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7
Color: Dark brown, black, to tan
Luster: Waxy to sub-vitreous

Identified March 16, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation in limestone. This process occurs in marine environments as silica replaces calcite. Geological age ranges from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools and weapons (flintknapping). Modern uses include road surfacing, jewelry (as jasper/agate variants), and as an abrasive in industrial applications.

Geological facts

Chert is so hard it can strike sparks against steel, which made it the primary material for 'flintlock' firearms. It is chemically identical to quartz but contains impurities that provide its opaque, earthy appearance.

Field identification & locations

Identify in the field by its smooth, waxy surface and 'clamshell-like' conchoidal fractures when chipped. It is significantly harder than steel and cannot be scratched by a pocket knife. Commonly found in limestone outcrops or as river-tumbled 'river jacks.'