igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 on Mohs scale. Color: Dark grey to black. Luster: Dull or sub-metallic. Crystal Structure: Fine-grained, aphanitic (microscopic crystals). Cleavage: None. Specific Gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull or sub-metallic
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Physical properties
Hardness: 6 on Mohs scale. Color: Dark grey to black. Luster: Dull or sub-metallic. Crystal Structure: Fine-grained, aphanitic (microscopic crystals). Cleavage: None. 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. Common in oceanic crust and volcanic regions. Age varies by location from Recent to Precambrian.
Uses & applications
Used extensively in construction as aggregate, road base, concrete, and railroad ballast. Also used for high-quality floor tiles and industrial stone wool insulation.
Geological facts
Basalt is the most common rock on the Earth's surface and makes up most of the ocean floor. It is also common on the Moon, Mars, and Venus.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture lacking visible crystals. Often found in volcanic fields, ancient lava flows, and near mid-ocean ridges. Collectors look for vesicular basalt (containing gas bubbles) or basalt with olivine inclusions.
