sedimentary

Chert or flint

Microcrystalline quartz (SiO₂), commonly chalcedony with possible opal or clay impurities

AI-generated identification · may be incorrect

Typically gray to brown, opaque, with a waxy or dull luster and very fine-grained structure; conchoidal fractures produce sharp edges. Hardness is about 7 Mohs, with no cleavage and specific gravity near 2.6.

Identified August 16, 2026More sedimentary →

Explore Chert or flint in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or cements sediments, often in limestone or chalk; many deposits range from Paleozoic to Cenozoic age. The specimen’s weathered, angular texture is consistent with chert, though the image alone cannot distinguish it confidently from some silicified rocks.

Uses & applications

Used historically for stone tools and fire-starting; today it is mainly a collector specimen, aggregate, or occasional decorative stone rather than a major industrial material.

Geological facts

Chert can preserve fossils and develop colorful bands or nodules. Flint is a dark, particularly fine-grained variety of chert traditionally associated with chalk formations.

Field identification & locations

Test for hardness by whether it scratches ordinary glass, and inspect for curved, shell-like fracture surfaces; a steel blade should not easily scratch it. Common locations include limestone and chalk regions worldwide, including the U.S. Midwest, Texas, England, and France.