Chert (Tan/Brown variety)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown; 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 radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens can range from Precambrian to recent ages.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in construction aggregate, and as a component in certain ceramics.

Geological Facts

Chert is so hard it can scratch glass. When struck against steel, it can produce sparks, leading to its historical use as a fire-starter and in flintlock firearms.

Field Identification & Locations

Identify by its 'waxy' appearance, extreme hardness, and distinctive shell-like (conchoidal) curved fracture patterns. Commonly found in riverbeds or as nodules within limestone outcrops.

Identified on: 5/3/2026

Mode: Standard