sedimentary

Iron-stained gray chert (flint) pebble

Microcrystalline quartz (SiO₂), likely chert with iron-oxide staining

AI-generated identification · may be incorrect

Dense, smooth, dark gray stone with ochre-brown iron staining and a waxy to slightly dull luster. Quartz hardness is about 7 Mohs; chert has no visible crystal structure or cleavage and commonly breaks with a curved, shell-like fracture. Exact identification is limited by the photo.

Identified September 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica precipitates or accumulates in sedimentary settings and later lithifies; iron oxides can stain weathered surfaces. Its age cannot be determined from appearance alone.

Uses & applications

Chert has historically been used for tools and fire-starting; attractive pieces may be polished or collected. This small, unverified specimen has no evident industrial value.

Geological facts

Chert is cryptocrystalline quartz: its crystals are too small to distinguish without magnification. Iron staining commonly produces yellow, orange, or rusty patches.

Field identification & locations

Look for a dense feel, smooth surface, and sharp, curved fracture; a steel point may scratch it only with difficulty. Chert occurs worldwide in sedimentary formations and as stream-worn pebbles.