Chert (Jasper type)

Physical Properties

Hardness: 6.5-7 Mohs; Color: Brown, olive-green, and black; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from hydrothermal fluids in sedimentary basins. Most common in ancient marine environments.

Uses & Applications

Historically used for making stone tools due to its sharp edges when fractured. Currently used in jewelry (as jasper), as decorative gravel, and for lapidary hobbies.

Geological Facts

Chert is so durable that it often survives long after the surrounding rock has eroded, which is why it is frequently found as pebbles on beaches or in stream beds like the one pictured.

Field Identification & Locations

Identify in the field by its hardness (it will scratch glass), its smooth 'conchoidal' (shell-like) fracture, and its waxy appearance. Commonly found in riverbeds and coastal beaches.

Identified on: 4/26/2026

Mode: Standard