igneous
Basalt (River Pebble)
Extrusive mafic igneous rock
Hardness: 6-7 Mohs; Color: Dark grey to charcoal black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0
- Hardness
- 6-7 Mohs
- Color
- Dark grey to charcoal black
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to charcoal black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth surface. This specific rounded specimen was shaped by fluvial erosion (river action) over thousands of years.
Uses & applications
Used extensively in construction as aggregate, for road base, railroad ballast, and in the production of stone wool insulation. Polished stones are used in landscaping and massage therapy.
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 and Mars.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture. Commonly found in volcanic regions, riverbeds, and coastal beaches. It will not react with acid unlike limestone.
