igneous
Basalt Cobble
Aphanitic Mafic Volcanic Rock (Basalt)
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-black; Luster: Dull (waxy when wet); Structure: Fine-grained/Aphanitic; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey to greenish-black
- Luster
- Dull (waxy when wet)
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-black; Luster: Dull (waxy when wet); Structure: Fine-grained/Aphanitic; 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. This specimen has been rounded and smoothed by fluvial or lacustrine erosion (water action).
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and sometimes in landscaping or as a polishing material.
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 on the Moon, Mars, and Venus.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and lack of visible crystals. It is ubiquitous in volcanic regions and riverbeds downstream from volcanic mountain ranges.
