Chert (Nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: Variegated grey, tan, and brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the replacement of limestone/chalk by silica in groundwater. Occurs throughout geological history from Precambrian to modern strata.

Uses & Applications

Historically used for making stone tools (arrowheads). Modernly used in road construction, as an abrasive, and sometimes in landscaping or lapidary work.

Geological Facts

Chert was one of the most important minerals for early humans because its conchoidal fracture creates edges sharper than a steel scalpel. Varieties of chert include flint and jasper.

Field Identification & Locations

Identify in the field by its hardness (will scratch glass and steel), waxy appearance on fresh surfaces, and the way it breaks into curved, shell-like flakes. Commonly found in limestone beds.

Identified on: 4/25/2026

Mode: Standard