sedimentary

Chert (specifically Tan/Gray Chert)

Microcrystalline quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs); Color: Tan, brownish-gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs)
Color: Tan, brownish-gray
Luster: Waxy to dull

Identified April 29, 2026More sedimentary →

Explore Chert (specifically Tan/Gray 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); Color: Tan, brownish-gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by the precipitation of silica from groundwater within sedimentary rock layers, often limestone.

Uses & applications

Historically used for stone tools (knapping) due to its sharp edges; used today in road construction aggregate and sometimes as a minor gemstone when colorful (Jasper).

Geological facts

Chert has a conchoidal fracture, which means it breaks like glass. This property made it the primary material for prehistoric arrowheads and scrapers.

Field identification & locations

Identify in the field by its hardness (it can scratch glass), waxy appearance, and shell-like fracture patterns. Common in limestone outcrops and riverbeds.