sedimentary
Sandstone with Desert Varnish
Arenite with Ferromanganese patina
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
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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.
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