Chert (Common Opal/Chalcedony mixture)

Physical Properties

Hardness: 6.5-7. Color: Tan, brown, grey, translucent cream. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Conchoidal fracture. Specific gravity: 2.6.

Formation & Geological History

Formed through the accumulation of silica-rich fluids in sedimentary basins or volcanic cavities. This specimen shows signs of secondary replacement of original host material by silica.

Uses & Applications

Historically used for stone tools (arrowheads). Modernly used as decorative landscaping stone or for lapidary tumbling.

Geological Facts

Chert is so durable that it can survive long periods of weathering, often being found in stream beds long after the surrounding rock has eroded. It produces sparks when struck against steel.

Field Identification & Locations

Identify in the field by its smooth, shell-like (conchoidal) fracture and its ability to scratch glass. Found globally in limestone layers or as nodules in various soils.

Identified on: 4/14/2026

Mode: Standard