sedimentary

Chert (Chocolate or Brown Flint)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Dark brown/chocolate; Luster: Waxy to dull; Crystal structure: Microcrystalline (Cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Dark brown/chocolate
Luster: Waxy to dull

Identified June 5, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Dark brown/chocolate; Luster: Waxy to dull; Crystal structure: Microcrystalline (Cryptocrystalline); Cleavage: None (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 chemical precipitation from groundwater within sedimentary layers like limestone or chalk. Many deposits date from the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (knapping) such as arrowheads. Currently used as road aggregate, in jewelry as lapidary material, and for spark-striking in primitive survival kits.

Geological facts

Chert is so hard it can scratch glass and steel. When it breaks, it forms razor-sharp edges because of its conchoidal (shell-like) fracture pattern, which made it the primary material for prehistoric survival.

Field identification & locations

Identify by its 'flinty' waxy texture, smooth curved fracture surfaces, and extreme hardness. It won't react with acid (unlike the limestone it often sits in). Commonly found as nodules or layers in carbonate rock outcrops.