sedimentary

Chert (Flint)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 (Mohs scale); Color: Grayish-blue with tan cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 7 (Mohs scale)
Color: Grayish-blue with tan cortex
Luster: Waxy to dull

Identified May 30, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 7 (Mohs scale); Color: Grayish-blue with tan cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & geological history

Formed primarily by the biochemical or chemical precipitation of silica. This often occurs in deep-sea environments from the accumulation of siliceous oozes or as nodules in limestone/chalk layers through groundwater replacement.

Uses & applications

Historically used for stone tools (knapping) and fire starting. Modern uses include abrasives, road construction aggregate, and occasionally as semi-precious gemstones in jewelry.

Geological facts

Chert is so hard it can scratch glass and steel. It was one of the most vital materials for early humans because its conchoidal fracture creates razor-sharp edges perfect for arrowheads and scrapers.

Field identification & locations

Identify in the field by its smooth, waxy texture on broken surfaces and characteristic shell-like (conchoidal) fracture patterns. It is commonly found as nodules within limestone or as river stones.