Chert (and Chalcedony)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Gray, white, brown with mottled patterns; Luster: Waxy to vitreous; Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.6

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in limestone or volcanic host rocks.

Uses & Applications

Historically used for stone tools (arrowheads); currently used for decorative landscaping, lapidary arts, and as a concrete aggregate.

Geological Facts

This specimen appears to be a common variety of chert with chalcedony inclusions. Chert was one of the most vital materials for early humans due to its ability to hold a razor-sharp edge when fractured.

Field Identification & Locations

Identify by its waxy texture, hardness (it will scratch glass), and shell-like (conchoidal) fracture pattern. Found globally in sedimentary basins and volcanic regions.

Identified on: 5/1/2026

Mode: Standard