igneous
Basalt with Desert Varnish
Mafic Igneous Rock with Ferromanganese Coating
Hardness: 6 (Mohs scale); Color: Grayish-green interior with a dark brown to black matte-to-submetallic coating; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Grayish-green interior with a dark brown to black matte-to-submetallic coating; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. The dark coating (desert varnish) is a secondary geological feature formed over thousands of years by the slow accumulation of minerals and microscopic organisms in arid environments.
Uses & applications
Used primarily in construction as crushed stone, road base, or railroad ballast. Specimens with heavy varnish are of interest to geologists and archaeologists.
Geological facts
Desert varnish grows very slowly, often at a rate of only a few micrometers per millennium. Ancient people used this varnish as a canvas for petroglyphs by scraping away the dark layer to reveal the lighter rock beneath.
Field identification & locations
Identify in the field by its high density, dark color, and the presence of a thin, dark, shiny or matte patina on exposed surfaces. Commonly found in volcanic fields and arid desert regions globally.
