sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale), Color: tan, grey, beige, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7

Identified June 23, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: tan, grey, beige, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the precipitation of silica from water or the accumulation of siliceous biological remains (like radiolaria) within limestone or chalk beds. Often Cretaceous or Jurassic in age.

Uses & applications

Historically used for stone tools (flintknapping). Modernly used as road metal, in construction aggregate, and as common garden stone.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture produces edges sharper than a steel scalpel. The rounded, bulbous shapes are often referred to as nodules.

Field identification & locations

Identify by its extreme hardness (scratches glass) and smooth, shell-like fracture pattern. Often found as rounded lumps in riverbeds or protruding from limestone outcrops.