sedimentary

Chert (with Manganese Dendrites)

Chert (Cryptocrystalline Quartz, SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs); Color: White to grey with dark brown/black surface staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65.

Hardness: 7 (Mohs)
Color: White to grey with dark brown/black surface staining
Luster: Waxy to dull

Identified April 7, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs); Color: White to grey with dark brown/black surface staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65.

Formation & geological history

Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) or chemical precipitation of silica in ocean floor sediments. The dark patterns are manganese oxides that precipitated into cracks or onto the surface.

Uses & applications

Historically used for stone tools/arrowheads. Currently used as industrial abrasive, landscaping gravel, and as educational or decorative lapidary specimens.

Geological facts

This specimen shows manganese oxide staining which often creates 'dendrites'—branching tree-like patterns that are frequently mistaken for plant fossils. Chert is the parent material for jasper and flint.

Field identification & locations

Identify by its hardness (will scratch glass), conchoidal (shell-like) fracture, and smooth, waxy texture. Commonly found in riverbeds and coastal regions where ancient sedimentary layers have eroded.