Chert (specifically Tan/Gray Chert)

Physical Properties

Hardness: 7 (Mohs); Color: Tan, brownish-gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by the precipitation of silica from groundwater within sedimentary rock layers, often limestone.

Uses & Applications

Historically used for stone tools (knapping) due to its sharp edges; used today in road construction aggregate and sometimes as a minor gemstone when colorful (Jasper).

Geological Facts

Chert has a conchoidal fracture, which means it breaks like glass. This property made it the primary material for prehistoric arrowheads and scrapers.

Field Identification & Locations

Identify in the field by its hardness (it can scratch glass), waxy appearance, and shell-like fracture patterns. Common in limestone outcrops and riverbeds.

Identified on: 4/29/2026

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