Chert (specifically Yellow/Tan Chert)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, yellow, or buff due to iron oxide impurities; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

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. Often Paleozoic to Mesozoic in age.

Uses & Applications

Historically used for stone tools (arrowheads). Modern uses include road surfacing, decorative landscaping mulch, and as a raw material for silicon production.

Geological Facts

Chert is so hard it can scratch glass and steel. It was one of the most important minerals for prehistoric humans because of its ability to produce a razor-sharp edge when fractured.

Field Identification & Locations

Identify in the field by its conchoidal (shell-like) fracture patterns and the fact that it cannot be scratched by a steel knife. Commonly found as nodules in limestone outcrops or as river pebbles.

Identified on: 4/19/2026

Mode: Standard