sedimentary

Chert / Flint Nodules in Limestone

Silicon Dioxide (SiO2) within Calcium Carbonate (CaCO3)

AI-generated identification · may be incorrect

Hardness: 7 (Chert) / 3 (Limestone); Color: Gray to Tan; Luster: Waxy to Dull; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture in chert); Specific Gravity: 2.6-2.7

Hardness: 7 (Chert) / 3 (Limestone)
Color: Gray to Tan
Luster: Waxy to Dull

Identified February 22, 2026More sedimentary →

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Physical properties

Hardness: 7 (Chert) / 3 (Limestone); Color: Gray to Tan; Luster: Waxy to Dull; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture in chert); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through chemical precipitation where silica replaces carbonate minerals in sedimentary beds, often in deep-sea environments or via groundwater during diagenesis.

Uses & applications

Used historically for stone tools (weapons/scrapers). Industrial uses include decorative landscaping, concrete aggregate, and silicon production.

Geological facts

Chert is so hard it can strike sparks from steel, which led to its use in flintlock firearms. It is chemically identical to quartz but contains microscopically small crystals.

Field identification & locations

Identify in the field by checking for a smooth, waxy texture that is much harder than the surrounding porous rock. Common in the midwestern USA and the Chalk cliffs of Europe. Collectors look for colorful variants like Jasper or Agate.