Chert (often called Flint or Jasper)

Physical Properties

Hardness: 7 on Mohs scale; Color: Brown, tan, and gray; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); 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 through the precipitation of silica in groundwater within limestone or chalk beds. It can range from Precambrian to Cenozoic in age.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate, road surfacing, and as a semi-precious gemstone when colorful.

Geological Facts

Because chert fractures in a conchoidal (shell-like) pattern with very sharp edges, it was the primary material for human survival for millions of years during the Stone Age. It produces sparks when struck against steel, leading to its use in flintlock firearms.

Field Identification & Locations

Identify by its waxy luster, hardness (it will scratch glass), and its characteristic smooth, curved fracture surfaces. Commonly found in stream beds or as nodules weathering out of limestone formations.

Identified on: 4/15/2026

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