igneous
Basalt with Weathering Crust
Aphanitic Mafic Igneous Rock
Hardness: 6 (Mohs); Color: Dark grey to black interior, brown/tan exterior (weathering rind); Luster: Dull to sub-metallic; Crystal Structure: Microcrystalline (aphanitic); Cleavage: None (conchoidal fracture).
- Hardness
- 6 (Mohs)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black interior, brown/tan exterior (weathering rind); Luster: Dull to sub-metallic; Crystal Structure: Microcrystalline (aphanitic); Cleavage: None (conchoidal fracture).
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 shows a distinct weathering rind from chemical exposure over time.
Uses & applications
Commonly used as aggregate in construction, road base, and concrete. High-quality specimens are used for architectural stone or as decorative garden landscape rocks.
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 heavy weight (high density), dark interior color, and lack of visible crystals. It often forms hexagonal columns in large outcrops.
