Chert (Nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Dark brown to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation within limestone or chalk beds during the Phanerozoic eon.

Uses & Applications

Historically used for stone tools and starting fires (flint); currently used in road construction, as an abrasive, and occasionally for lapidary work or aquarium decor.

Geological Facts

Dark varieties of chert are often specifically called 'flint.' It is so hard that it can scratch glass and produces sparks when struck against steel.

Field Identification & Locations

Identify by its 'shell-like' conchoidal fracture patterns on broken surfaces and its extreme hardness. Commonly found in riverbeds or as nodules inside sedimentary limestone outcrops.

Identified on: 4/22/2026

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