Basalt with Desert Varnish

Hardness: 6 (Mohs scale), Color: Dark grey to black with reddish-brown oxidation, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The outer coloring is due to 'desert varnish'—a thin coating of manganese, iron oxides, and clay minerals formed over centuries in arid environments.

Primarily used in construction for road base, concrete aggregate, and railroad ballast. Weathered specimens are of interest to geologists studying paleoclimates.

The dark reddish-brown coating seen here is a biological and chemical process that can take thousands of years to develop in desert climates. It was often used by ancient cultures to create petroglyphs by scratching through the dark layer.

Identify in the field by its heavy weight (high density), dark interior color when broken, and fine-grained texture. Commonly found in volcanic fields, oceanic crust, and plateaus.