Chert (Nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed by the precipitation of silica from groundwater or the accumulation of siliceous organic remains (like diatoms or radiolarians) within sedimentary layers, typically limestone or chalk.

Uses & Applications

Historically used for making stone tools (arrowheads). Currently used in road construction, as jewelry (when patterned), and as high-silica industrial raw material.

Geological Facts

Chert is so hard it can scratch glass and steel. It was one of the most important materials for early humans because its conchoidal fracture creates razor-sharp edges.

Field Identification & Locations

Identify in the field by its waxy appearance, extreme hardness, and 'shell-like' curved fractures (conchoidal). It is common in riverbeds and limestone outcrops.

Identified on: 4/26/2026

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