sedimentary
Desert Varnish on Sandstone
Quartz Arenite with Iron-Manganese Oxide Coating
Hardness: 6.5-7 (sandstone matrix), Color: orange-brown to near black, Luster: dull to sub-metallic, Texture: porous under-layer with a thin, hard mineral skin
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (sandstone matrix), Color: orange-brown to near black, Luster: dull to sub-metallic, Texture: porous under-layer with a thin, hard mineral skin
Formation & geological history
Formed over thousands of years as manganese and iron oxides are deposited by bacteria or wind-borne dust and then cemented to rock surfaces in arid environments. Typically found on Paleozoic or Mesozoic sandstones.
Uses & applications
Geological study, archaeological dating (petroglyphs), and desert landscape aesthetics.
Geological facts
Desert varnish is often only micrometers thick, yet it can be thousands of years old. Ancient civilizations used it as a canvas by scratching through the dark varnish to reveal the light rock underneath, creating petroglyphs.
Field identification & locations
Identify by the dark, often shiny or leathery thin coating on exposed desert rock faces. It is common in the Southwestern United States, Northern Africa, and the Middle East.
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