sedimentary

Chert with Desert Varnish

Cryptocrystalline Quartz (SiO2) with Manganese/Iron oxide coating

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Dark brown to black outer coating, lighter interior; Luster: Waxy to dull; Crystal structure: Microcrystalline; Conchoidal fracture; Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark brown to black outer coating, lighter interior
Luster: Waxy to dull

Identified May 14, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Dark brown to black outer coating, lighter interior; Luster: Waxy to dull; Crystal structure: Microcrystalline; Conchoidal fracture; Specific gravity: 2.65.

Formation & geological history

Formed as a microcrystalline sedimentary rock in marine or terrestrial environments. The dark 'varnish' is a thin coating formed over thousands of years by wind-blown dust and biochemical processes in arid environments.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracture. Today used as decorative landscaping stone or as a study specimen for desert geomorphology.

Geological facts

Desert varnish grows incredibly slowly, often less than a millimeter every few thousand years. It is often used by archaeologists to date petroglyphs carved into the dark surface.

Field identification & locations

Identify by its very hard, quartz-like interior and dark, manganese-rich outer crust. Found commonly in arid regions like the American Southwest. It will scratch glass easily.