Chert (Jasper/Agate variety)

Physical Properties

Hardness: 6.5–7 (Mohs scale); Color: Tan, beige, and pinkish-white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains or chemical precipitation of silica in sedimentary environments like deep sea beds or limestone replacements.

Uses & Applications

Historically used for stone tools due to sharp fracture edges; currently used for road base, landscaping, and lapidary art as tumbled stones.

Geological Facts

Chert has been used by humans for hundreds of thousands of years to create arrowheads, spear points, and scrapers because it breaks in predictable, razor-sharp curves.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and as nodules in limestone/chalk.

Identified on: 4/17/2026

Mode: Standard