igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 Mohs; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained), often containing micro-crystals of plagioclase feldspar and pyroxene; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained), often containing micro-crystals of plagioclase feldspar and pyroxene; 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 surface of a terrestrial planet or moon. Most terrestrial basalt formed during the Cenozoic era, though it can date back billions of years.
Uses & applications
Extensively used in construction as aggregate (crushed stone) for road base, concrete, and asphalt; used in industrial high-temp insulation (rock wool); occasionally polished for decorative river stones and jewelry.
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 the primary rock type found on the Moon (the dark 'seas' or Maria) and Mars.
Field identification & locations
Identify in the field by its heavy weight (density), dark color, and lack of visible large crystals. Commonly found in volcanic regions, oceanic islands (like Hawaii), and riverbeds near volcanic plateaus. Collectors look for vesicles (gas bubbles) or olivine inclusions.
