Chert (often called Flint when dark)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white with some green/brown staining; Luster: Dull to waxy; Crystal Structure: Microcrystalline; Cleavage: None (shows 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 by the replacement of carbonate material in sedimentary beds. Most common in limestone or chalk deposits ranging from Precambrian to Cenozoic eras.

Uses & Applications

Historically used for stone tools and weapons (arrowheads). Modernly used as an abrasive, in construction aggregate, and as a starting material for roadbeds.

Geological Facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel. It was the primary material for the Stone Age technology revolution.

Field Identification & Locations

Identify by its hardness (it will scratch glass) and its smooth, curved 'shell-like' fracture patterns. Often found as nodules or layers within limestone. Collectors look for unique colors or fossil inclusions.

Identified on: 5/8/2026

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