sedimentary

Chert (often called Flint when dark)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Cream, white, Tan, or brown with possible banding; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (displays conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Cream, white, Tan, or brown with possible banding
Luster: Waxy to dull

Identified April 17, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the seafloor, which then undergo diagenesis. It can also form as nodules in limestone through chemical replacement.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; modern uses include road surfacing, gemstones, and as a component in glass manufacturing.

Geological facts

Because of its high hardness and brittle nature, prehistoric humans used it as their primary material for tools and weapons. It can strike a spark against steel, making it vital for fire-starting throughout history.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) breaks that result in very sharp edges. It will not be scratched by a steel knife. Commonly found in limestone beds or as water-worn pebbles in stream beds.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite

sedimentary