sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Black, grey, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Black, grey, or tan
Luster: Waxy to dull

Identified March 15, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Black, grey, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from groundwater or within carbonate rocks like limestone. Often occurs in Paleozoic or Mesozoic era deposits where silica replaces organic material or fills voids.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; modern uses include road construction aggregate and decorative landscaping rocks.

Geological facts

Chert is so hard it can strike sparks from steel, a common way it was used to start fires in the past. It is often found as rounded nodules or flat layers within chalk or limestone formations.

Field identification & locations

Identify by its 'conchoidal' (shell-like) fracture pattern and its ability to scratch glass. Commonly found in riverbeds, limestone quarries, and coastal chalk cliffs. Look for smooth, shell-like curves on broken surfaces.