sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, brown, or buff with a weathered exterior (cortex); Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None, exhibits conchoidal fracture; Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Tan, brown, or buff with a weathered exterior (cortex)
Luster: Waxy to dull

Identified April 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. It can also form from the accumulation of siliceous skeletal remains like diatoms or radiolarians. Most common in Paleozoic and Mesozoic strata.

Uses & applications

Historically used for stone tools and weapons (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate, road metal, and as a starting material for silicon. High-quality varieties are used for lapidary work and jewelry.

Geological facts

Chert is so hard it can strike sparks from steel, which led to its use in 'flintlock' firearms. It is incredibly durable and chemically resistant to weathering, which is why it often remains as pebbles long after the surrounding limestone has eroded.

Field identification & locations

Identify in the field by its conchoidal (shell-like) fracture pattern and its ability to scratch glass. Often found as nodules or 'layers' in limestone outcrops. Collectors should look for the smooth, waxy interior surface revealed beneath the rough, chalky outer rind.