Chert (Chalcedony/Agate)

Physical Properties

Hardness: 6.5-7 Mohs; Color: Brownish-yellow, tan, and translucent white; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65

Formation & Geological History

Formed via the precipitation of silica from groundwater or within sedimentary rock layers like limestone. This specimen appears water-worn, likely tumbled in a river or coastal environment over thousands of years.

Uses & Applications

Historically used for making stone tools; currently used for decorative stone, lapidary work, jewelry (when polished), and aggregate in construction.

Geological Facts

This specific sample shows clear waxy luster and translucency typical of chalcedony. Chert is so durable that it often survives long after the surrounding rock has eroded away.

Field Identification & Locations

Identified by its waxy texture, lack of visible crystals, and conchoidal (shell-like) fracture patterns. Found globally in gravel beds, beaches, and sedimentary basins.

Identified on: 5/3/2026

Mode: Standard