sedimentary

Chert (with Limestone matrix)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, black, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, black, tan
Luster: Waxy to dull

Identified March 21, 2026More sedimentary →

Explore Chert (with Limestone matrix) 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 on Mohs scale; Color: Gray, black, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the precipitation of silica in groundwater within sedimentary host rocks like limestone during various geological eras.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, construction aggregate, and occasionally as an ornamental garden stone.

Geological facts

Chert was one of the most vital resources for prehistoric humans, often traded over long distances to produce arrowheads and scrapers. It can spark when struck against steel.

Field identification & locations

Identify in the field by its hardness (will scratch glass), smooth waxy feel, and curved 'eggshell' breaks. Found globally in limestone and chalk beds, often appearing as nodules or layers.