sedimentary
Sandstone with Desert Varnish
Arenite (SiO2 predominantly) with Ferromanganese coating
Hardness: 6-7 (Mohs scale for the host rock); Color: Tan/brown host with dark brown to black metallic surface coating; Luster: Dull to sub-metallic; Crystal Structure: Clastic granular texture; Specific Gravity: 2.2-2.8
- Hardness
- 6-7 (Mohs scale for the host rock)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6-7 (Mohs scale for the host rock); Color: Tan/brown host with dark brown to black metallic surface coating; Luster: Dull to sub-metallic; Crystal Structure: Clastic granular texture; Specific Gravity: 2.2-2.8
Formation & geological history
Formed via the lithification of sand particles over millions of years. The surface 'varnish' is a thin coating of manganese/iron oxides and clay minerals deposited by wind and microbes in arid environments.
Uses & applications
Host rock used in construction and paving. The varnished surface is historically vital for creating petroglyphs (rock art).
Geological facts
Desert varnish takes thousands of years to form and can be used to date archaeological sites. It is often only a few hundred micrometers thick but is incredibly stable.
Field identification & locations
Identify by the dark, shiny, or metallic sheen on the surface of a lighter-colored rock in desert regions. Common in the Southwestern United States, Sahara, and Australian outback.
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