igneous
Basalt (with possible weathering)
Basalt
Hardness: 5-6 (Mohs), Color: Dark grey to greenish-grey, Luster: Dull/Sub-metallic (weathered), Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 5-6 (Mohs), Color: Dark grey to greenish-grey, Luster: Dull/Sub-metallic (weathered), 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 or very near the Earth's surface. This specimen appears to have undergone minor surface weathering/oxidation producing the brownish spots.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and in the manufacturing of stone wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also extremely common on the Moon and Mars.
Field identification & locations
Identified by its heavy weight (density), dark color, and fine grain size. Often found in volcanic regions or as river stones transported from volcanic highlands.
