Chert (Nodule)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Brown, grey, tan, or black; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone/chalk by silica-heavy groundwater. Found globally in sedimentary strata ranging from the Precambrian to Cenozoic eras.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as road surfacing material, in construction aggregate, and occasionally as high-end decorative stones when polished.

Geological Facts

Chert and its variety, flint, were the first materials used by early humans to create fire and weaponry. The specimen in the image shows rhythmic surface patterns that may be natural conchoidal fracturing or weathered fossil impressions common in nodules.

Field Identification & Locations

Identify in the field by its hardness (it will scratch glass), its waxy feel, and its curved, glass-like fracture surfaces. Often found as rounded nodules or continuous beds within limestone or chalk outcrops.

Identified on: 4/17/2026

Mode: Standard