sedimentary

Chert (Nodular)

Microcrystalline Silica (SiO2)

Hardness: 6.5 - 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65

Hardness: 6
Color: Gray, brown, or tan
Luster: Dull to waxy

Identified March 25, 2026More sedimentary →

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Physical properties

Hardness: 6.5 - 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. Often Paleozoic to Mesozoic in age.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction aggregate and occasionally as a gemstone (Jasper varieties).

Geological facts

Chert is so hard it can create sparks when struck against steel. Prehistoric humans highly valued it for its ability to fracture into extremely sharp edges for arrowheads and knives.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its shell-like (conchoidal) fracture pattern, and its typical occurrence as rounded nodules within softer sedimentary rock like limestone. Common globally in karst regions.