igneous
Basalt with Desert Varnish
Mafic Volcanic Rock (Fe/Mg rich Silicate)
Hardness: 6 (Mohs scale), Color: Dark grey to black with reddish-brown oxidation, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black with reddish-brown oxidation, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), 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 is 'desert varnish' formed by manganese and iron oxides interacting with clay minerals and microbes over thousands of years.
Uses & applications
Used primarily in construction for crushed stone, road base, and concrete aggregate. In landscaping, it is used for decorative rock. High-quality smooth pieces may be used for hot stone massage therapy.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The dark 'varnish' seen on these rocks in arid environments was often used by ancient peoples to create petroglyphs by chipping through the dark layer.
Field identification & locations
Identify by its heavy weight, dark interior, and fine-grained texture. Commonly found in volcanic fields, rift valleys, and oceanic plates. Look for 'vesicles' (gas bubbles) or columnar jointing in larger outcrops.
