Chert (Jasper/Agate variety)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Opaque reddish-brown, tan, and gray. Luster: Waxy to dull. Crystal Structure: Hexagonal (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.

Formation & Geological History

Formed primarily through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary environments. Common in marine limestone sequences or as hydrothermal replacements.

Uses & Applications

Historically used for stone tools due to its sharp edges; modernly used for lapidary work, jewelry (when colorful like Jasper), and as a minor gemstone for collectors.

Geological Facts

Chert has been used by humans for millions of years to create arrowheads and scrapers. Because it breaks with a conchoidal (shell-like) fracture, it produces very sharp, glass-like edges.

Field Identification & Locations

Identify in the field by its hardness (it will scratch glass or steel) and its smooth, waxy fracture surface. Look for it in riverbeds or within limestone and dolomite outcrops.

Identified on: 4/29/2026

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