igneous
Basalt (with possible minor veining)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs), Color: Charcoal grey to dark green, Luster: Dull to sub-metallic on fresh surfaces, Crystal Structure: Aphanitic (fine-grained), Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 6 (Mohs), Color: Charcoal grey to dark green, Luster: Dull to sub-metallic on fresh surfaces, 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 (mafic lava) at or near the Earth's surface. This piece appears to be a stream-rounded or weathered cobble.
Uses & applications
Used primarily in construction for road base, concrete aggregate, railroad ballast, and ground cover. High-quality dense basalt is also used for dimensional stone or flooring.
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 found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its weight (dense for its size), dark color, and fine-grained texture. It often shows smooth, rounded edges when found in riverbeds or gravel pits.
