igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black or dark greenish-brown; Luster: Dull (aphanitic texture); Crystal Structure: Fine-grained, often containing microscopic plagioclase feldspar and pyroxene; Cleavage: None/Irregular fracture; Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black or dark greenish-brown
- Luster
- Dull (aphanitic texture)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black or dark greenish-brown; Luster: Dull (aphanitic texture); Crystal Structure: Fine-grained, often containing microscopic plagioclase feldspar and pyroxene; Cleavage: None/Irregular fracture; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) exposed at or very near the Earth's surface. Can range from modern eruptions to billions of years old; found in oceanic crust and volcanic hotspots.
Uses & applications
Extensively used in construction as aggregate for road base, concrete, and railroad ballast. Also used as a dimension stone for floor tiles and in the production of stone wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also common on other planetary bodies like the Moon (lunar maria), Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible large crystal grains. Commonly found in riverbeds (where it becomes rounded, as seen in the image) or near volcanic sites. Collectors look for vesicular basalt (with gas bubble holes) or specimens containing green olivine crystals.
