Chert (Jasper variety)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and grey streaks; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich remains of microscopic organisms (like radiolaria) or via chemical precipitation from silica-rich fluids in sedimentary basins. This specimen likely dates from the Paleozoic to Mesozoic era depending on the local strata.

Uses & Applications

Historically used for stone tools (knapping) due to its sharp edges; currently used for decorative landscaping, jewelry (when polished as jasper), and as an abrasive in some industrial applications.

Geological Facts

Chert is so tough and chemically stable that it often remains as 'river jacks' or gravel long after the surrounding rock has weathered away. It is one of the primary materials used by prehistoric humans for arrowheads and scrapers.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. It is commonly found in limestone or chalk beds as nodules or layers.

Identified on: 5/1/2026

Mode: Standard