sedimentary

Chert (Iron-stained)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, and black due to iron/manganese oxides; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, and black due to iron/manganese oxides
Luster: Waxy to dull

Identified March 27, 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 through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids within limestone or chalk beds. This specimen shows a weathered 'patina' or cortex on the exterior.

Uses & applications

Historically used for stone tools/weapons due to conchoidal fracture. Modern uses include road surfacing, decorative landscaping, and occasionally as a gemstone when colorful (Jasper).

Geological facts

Chert is so durable that it often survives as 'river gravel' long after the surrounding limestone host rock has eroded away. Iron-stained varieties are common in glacial till and riverbeds.

Field identification & locations

Identify by its extreme hardness (will scratch glass) and smooth, shell-like (conchoidal) fracture surfaces when broken. Look for the white/creamy interior contrasting with the stained outer 'skin'. Commonly found in riverbeds and gravel pits.