Chert (Nodular)

Physical Properties

Hardness: 7 on Mohs scale; Color: Brown, tan, and gray; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & Geological History

Formed through the replacement of limestone or carbonate sediment by silica-rich fluids (secondary replacement) or by the accumulation of siliceous remains of organisms like diatoms and sponges. Common in Paleozoic and Mesozoic sedimentary strata.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative landscaping rock, in construction aggregate, and occasionally for lapidary work.

Geological Facts

Chert is so hard that it can strike sparks against steel, a property used by ancient people to start fires. Flint is a high-quality variety of chert often associated with chalk.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel blade), smooth waxy texture on fresh breaks, and curved conchoidal fracture patterns. Found in riverbeds and limestone outcrops.

Identified on: 4/20/2026

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