Chert (Common Opal variant)

Physical Properties

Hardness: 6.5-7 Mohs; Color: White to creamy tan with waxy luster; Crystal Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & Geological History

Formed from the accumulation of silica-rich fluids in sedimentary basins or the replacement of limestone/dolomite. Often results from the remains of diatoms or radiolarians. Age can range from Precambrian to Cenozoic.

Uses & Applications

Historically used for stone tools (knapping); currently used in jewelry as cabochons, as an abrasive, and by rock collectors.

Geological Facts

Chert and its variants have been the primary material for arrowheads and scrapers for thousands of years due to their razor-sharp conchoidal fracture edges.

Field Identification & Locations

Identify in the field by its waxy appearance, inability to be scratched by steel, and 'shell-like' curved breaks. Commonly found in limestone beds and riverbeds.

Identified on: 4/13/2026

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