sedimentary

Chert (specifically Flint or Dark Chert)

Silicon Dioxide (SiO2) - Microcrystalline Quartz

AI-generated identification · may be incorrect

Hardness: 7 Mohs scale. Color: Black, dark gray, with tan/cream cortex. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.

Hardness: 7 Mohs scale
Color: Black, dark gray, with tan/cream cortex
Luster: Waxy to dull

Identified July 9, 2026More sedimentary →

Explore Chert (specifically Flint or Dark Chert) in the encyclopedia →

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: Black, dark gray, with tan/cream cortex. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.

Formation & geological history

Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) within sedimentary layers, typically limestone or chalk. Can be millions of years old, often dating from the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to sharp edges; used in fire-starting (flint and steel); modern uses include abrasives and road surfacing.

Geological facts

This specimen appears to be a 'flake' or debitage, possibly resulting from prehistoric tool-making (knapping). The characteristic 'cortex' (chalky outer layer) is visible at the top.

Field identification & locations

Identify in the field by its shell-like (conchoidal) fracture pattern and its ability to scratch glass. Commonly found in limestone outcrops or as weathered nodules in riverbeds.