Chert (specifically Flint or Common Opal/Chalcedony mix)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, and grey; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich fluids in sedimentary basins or by the replacement of carbonate rocks. Often found in nodules within limestone or chalk formations.

Uses & Applications

Used historically for stone tools and weapons (knapping); modern uses include construction aggregate, road surfacing, and as a decorative stone for lapidary work.

Geological Facts

Chert is one of the most durable rocks on Earth. Because it fractures into sharp edges, it was the primary material for prehistoric arrowheads and spear tips. When struck against steel, it can produce sparks due to its hardness.

Field Identification & Locations

Identify in the field by its distinctive conchoidal (shell-like) fracture patterns and waxy texture. It is harder than steel and will easily scratch glass. Common in riverbeds and limestone outcrops.

Identified on: 4/15/2026

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