sedimentary

Sandstone with Desert Varnish

Arenite with Ferromanganese patina

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Color: Tan/Grey matrix with dark brown to black coating; Luster: Dull/Earthy; Structure: Clastic; Cleavage: None

Hardness: 6-7 (Mohs)
Color: Tan/Grey matrix with dark brown to black coating
Luster: Dull/Earthy

Identified June 13, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs); Color: Tan/Grey matrix with dark brown to black coating; Luster: Dull/Earthy; Structure: Clastic; Cleavage: None

Formation & geological history

Formed through the lithification of sand particles over millions of years. The dark coating (Desert Varnish) is a thin layer of manganese and iron oxides formed by biochemical interaction with microbes in arid environments.

Uses & applications

Construction material, landscaping, and geological study. Historically used by indigenous people for petroglyphs (rock carvings).

Geological facts

Desert varnish grows incredibly slowly, often less than 1 micrometer per century. It consists largely of clay minerals mixed with iron and manganese oxides.

Field identification & locations

Identify by the contrast between the dark, stained outer surface and the lighter interior matrix. Commonly found in the Southwestern United States, Australian outback, and the Sahara desert.