Chert (Jasper)

Physical Properties

Hardness: 6.5-7 (Mohs); Color: Dark brown/black with reddish-brown patches; Luster: Waxy to dull (glassy when wet); Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.6

Formation & Geological History

Formed through the chemical precipitation of silica-rich fluids in sedimentary environments or as a replacement of organic material. These specimens are often reworked by water movement into rounded pebbles.

Uses & Applications

Historically used for stone tools and weapons (flint knapping); currently used as river rock for landscaping, in aggregate, or as semi-precious gemstones for jewelry (cabs).

Geological Facts

Chert and its variety Jasper were critical to human survival for thousands of years because they break with a conchoidal fracture, creating sharp edges for knives and arrowheads.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, shell-like fracture marks. Found globally in riverbeds, beaches, and sedimentary layers.

Identified on: 4/28/2026

Mode: Standard