sedimentary

Chert / Flint

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Dark brown, black, or tan with waxy luster; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark brown, black, or tan with waxy luster

Identified March 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Dark brown, black, or tan with waxy luster; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds from various geological ages, notably the Cretaceous period.

Uses & applications

Historically used for stone tools (arrowheads, knives) and strike-a-lights. Today used as a gemstone (Jasper/Flint variety), in road construction, and for aquarium or landscaping decor.

Geological facts

Because it breaks with a conchoidal (shell-like) fracture creating very sharp edges, it was the primary material for the Stone Age tool industry. It can spark when struck against steel.

Field identification & locations

Identify by its waxy appearance, extreme hardness (scratches glass), and translucent edges when held to light. Commonly found in riverbeds, glacial till, or embedded in carbonate rocks.